Name: Kiomars Nassiri K. CEE 517 (Traffic Signal Systems) Term Paper
UIN: 674851811 Interconnected Signals at Railroad Cro...
Name: Kiomars Nassiri K. CEE 517 (Traffic Signal Systems) Term Paper
UIN: 674851811 Interconnected Signals at Railroad Cro...
Name: Kiomars Nassiri K. CEE 517 (Traffic Signal Systems) Term Paper
UIN: 674851811 Interconnected Signals at Railroad Cro...
Name: Kiomars Nassiri K. CEE 517 (Traffic Signal Systems) Term Paper
UIN: 674851811 Interconnected Signals at Railroad Cro...
Name: Kiomars Nassiri K. CEE 517 (Traffic Signal Systems) Term Paper
UIN: 674851811 Interconnected Signals at Railroad Cro...
Name: Kiomars Nassiri K. CEE 517 (Traffic Signal Systems) Term Paper
UIN: 674851811 Interconnected Signals at Railroad Cro...
Name: Kiomars Nassiri K. CEE 517 (Traffic Signal Systems) Term Paper
UIN: 674851811 Interconnected Signals at Railroad Cro...
Name: Kiomars Nassiri K. CEE 517 (Traffic Signal Systems) Term Paper
UIN: 674851811 Interconnected Signals at Railroad Cro...
Name: Kiomars Nassiri K. CEE 517 (Traffic Signal Systems) Term Paper
UIN: 674851811 Interconnected Signals at Railroad Cro...
Name: Kiomars Nassiri K. CEE 517 (Traffic Signal Systems) Term Paper
UIN: 674851811 Interconnected Signals at Railroad Cro...
Name: Kiomars Nassiri K. CEE 517 (Traffic Signal Systems) Term Paper
UIN: 674851811 Interconnected Signals at Railroad Cro...
Name: Kiomars Nassiri K. CEE 517 (Traffic Signal Systems) Term Paper
UIN: 674851811 Interconnected Signals at Railroad Cro...
Name: Kiomars Nassiri K. CEE 517 (Traffic Signal Systems) Term Paper
UIN: 674851811 Interconnected Signals at Railroad Cro...
Name: Kiomars Nassiri K. CEE 517 (Traffic Signal Systems) Term Paper
UIN: 674851811 Interconnected Signals at Railroad Cro...
Name: Kiomars Nassiri K. CEE 517 (Traffic Signal Systems) Term Paper
UIN: 674851811 Interconnected Signals at Railroad Cro...
Name: Kiomars Nassiri K. CEE 517 (Traffic Signal Systems) Term Paper
UIN: 674851811 Interconnected Signals at Railroad Cro...
Name: Kiomars Nassiri K. CEE 517 (Traffic Signal Systems) Term Paper
UIN: 674851811 Interconnected Signals at Railroad Cro...
Name: Kiomars Nassiri K. CEE 517 (Traffic Signal Systems) Term Paper
UIN: 674851811 Interconnected Signals at Railroad Cro...
Name: Kiomars Nassiri K. CEE 517 (Traffic Signal Systems) Term Paper
UIN: 674851811 Interconnected Signals at Railroad Cro...
Name: Kiomars Nassiri K. CEE 517 (Traffic Signal Systems) Term Paper
UIN: 674851811 Interconnected Signals at Railroad Cro...
Name: Kiomars Nassiri K. CEE 517 (Traffic Signal Systems) Term Paper
UIN: 674851811 Interconnected Signals at Railroad Cro...
Name: Kiomars Nassiri K. CEE 517 (Traffic Signal Systems) Term Paper
UIN: 674851811 Interconnected Signals at Railroad Cro...
Name: Kiomars Nassiri K. CEE 517 (Traffic Signal Systems) Term Paper
UIN: 674851811 Interconnected Signals at Railroad Cro...
Name: Kiomars Nassiri K. CEE 517 (Traffic Signal Systems) Term Paper
UIN: 674851811 Interconnected Signals at Railroad Cro...
Name: Kiomars Nassiri K. CEE 517 (Traffic Signal Systems) Term Paper
UIN: 674851811 Interconnected Signals at Railroad Cro...
Name: Kiomars Nassiri K. CEE 517 (Traffic Signal Systems) Term Paper
UIN: 674851811 Interconnected Signals at Railroad Cro...
Name: Kiomars Nassiri K. CEE 517 (Traffic Signal Systems) Term Paper
UIN: 674851811 Interconnected Signals at Railroad Cro...
Name: Kiomars Nassiri K. CEE 517 (Traffic Signal Systems) Term Paper
UIN: 674851811 Interconnected Signals at Railroad Cro...
Name: Kiomars Nassiri K. CEE 517 (Traffic Signal Systems) Term Paper
UIN: 674851811 Interconnected Signals at Railroad Cro...
Name: Kiomars Nassiri K. CEE 517 (Traffic Signal Systems) Term Paper
UIN: 674851811 Interconnected Signals at Railroad Cro...
Name: Kiomars Nassiri K. CEE 517 (Traffic Signal Systems) Term Paper
UIN: 674851811 Interconnected Signals at Railroad Cro...
Name: Kiomars Nassiri K. CEE 517 (Traffic Signal Systems) Term Paper
UIN: 674851811 Interconnected Signals at Railroad Cro...
Name: Kiomars Nassiri K. CEE 517 (Traffic Signal Systems) Term Paper
UIN: 674851811 Interconnected Signals at Railroad Cro...
Name: Kiomars Nassiri K. CEE 517 (Traffic Signal Systems) Term Paper
UIN: 674851811 Interconnected Signals at Railroad Cro...
Name: Kiomars Nassiri K. CEE 517 (Traffic Signal Systems) Term Paper
UIN: 674851811 Interconnected Signals at Railroad Cro...
Name: Kiomars Nassiri K. CEE 517 (Traffic Signal Systems) Term Paper
UIN: 674851811 Interconnected Signals at Railroad Cro...
Name: Kiomars Nassiri K. CEE 517 (Traffic Signal Systems) Term Paper
UIN: 674851811 Interconnected Signals at Railroad Cro...
Name: Kiomars Nassiri K. CEE 517 (Traffic Signal Systems) Term Paper
UIN: 674851811 Interconnected Signals at Railroad Cro...
Name: Kiomars Nassiri K. CEE 517 (Traffic Signal Systems) Term Paper
UIN: 674851811 Interconnected Signals at Railroad Cro...
Name: Kiomars Nassiri K. CEE 517 (Traffic Signal Systems) Term Paper
UIN: 674851811 Interconnected Signals at Railroad Cro...
Name: Kiomars Nassiri K. CEE 517 (Traffic Signal Systems) Term Paper
UIN: 674851811 Interconnected Signals at Railroad Cro...
Name: Kiomars Nassiri K. CEE 517 (Traffic Signal Systems) Term Paper
UIN: 674851811 Interconnected Signals at Railroad Cro...
Name: Kiomars Nassiri K. CEE 517 (Traffic Signal Systems) Term Paper
UIN: 674851811 Interconnected Signals at Railroad Cro...
Name: Kiomars Nassiri K. CEE 517 (Traffic Signal Systems) Term Paper
UIN: 674851811 Interconnected Signals at Railroad Cro...
Name: Kiomars Nassiri K. CEE 517 (Traffic Signal Systems) Term Paper
UIN: 674851811 Interconnected Signals at Railroad Cro...
Name: Kiomars Nassiri K. CEE 517 (Traffic Signal Systems) Term Paper
UIN: 674851811 Interconnected Signals at Railroad Cro...
Name: Kiomars Nassiri K. CEE 517 (Traffic Signal Systems) Term Paper
UIN: 674851811 Interconnected Signals at Railroad Cro...
Name: Kiomars Nassiri K. CEE 517 (Traffic Signal Systems) Term Paper
UIN: 674851811 Interconnected Signals at Railroad Cro...
Name: Kiomars Nassiri K. CEE 517 (Traffic Signal Systems) Term Paper
UIN: 674851811 Interconnected Signals at Railroad Cro...
Name: Kiomars Nassiri K. CEE 517 (Traffic Signal Systems) Term Paper
UIN: 674851811 Interconnected Signals at Railroad Cro...
of 50

Kiomars Nassiri-CEE 517 Term paper (Final Submission)

Published on: Mar 3, 2016
Source: www.slideshare.net


Transcripts - Kiomars Nassiri-CEE 517 Term paper (Final Submission)

  • 1. Name: Kiomars Nassiri K. CEE 517 (Traffic Signal Systems) Term Paper UIN: 674851811 Interconnected Signals at Railroad Crossings 0 | P a g e Kiomars Nassiri K. UIN: 674851811 CEE 517- Traffic Signal Systems Term Paper Spring 2015 Interconnected Signals at Railroad Crossings
  • 2. Name: Kiomars Nassiri K. CEE 517 (Traffic Signal Systems) Term Paper UIN: 674851811 Interconnected Signals at Railroad Crossings 1 | P a g e Table of Contents Introduction ......................................................................................................................................5 What are the objectives of the paper? ...............................................................................................7 What are the important topics? .........................................................................................................8 Main Issues with this configuration ..............................................................................................8 When to Preempt?.......................................................................................................................9 Warning time.............................................................................................................................10 Safety Issues with preemption system in place at intersections...................................................10 Track clearance time..................................................................................................................11 Installing Pre-signals ..................................................................................................................11 What is the method/technique/concept behind it (them)? ...............................................................12 Train Detection Systems.............................................................................................................12 Fixed Distance systems ........................................................................................................13 Constant warning time-controlled track circuits (CWT)..........................................................14 Other non-track-based systems ...........................................................................................15 Highway-rail grade crossing warning devices and systems ..........................................................15 Flashing Light Signals and Bells ............................................................................................16 Automatic Gates (Boom Barriers): .......................................................................................16 Other components ..............................................................................................................17 Highway traffic signals near highway-rail grade crossings............................................................17 Requirements of Interconnection ........................................................................................17 When to preempt traffic signals ..........................................................................................18 Characteristics of different types of preempted signal controllers ........................................18 Interconnection Circuitry .....................................................................................................18 Preemption Phasing and Sequence:......................................................................................19 Entry into preemption mode: ............................................................................................19 Termination of the current interval in operation (right-of-way transfer) ........................20 Initiation of “clear track” intervals ................................................................................23 1. Number of track clearance intervals .......................................................23 2. Track clearance signal indications displayed ...........................................23 3. Clearance Configuration ........................................................................23
  • 3. Name: Kiomars Nassiri K. CEE 517 (Traffic Signal Systems) Term Paper UIN: 674851811 Interconnected Signals at Railroad Crossings 2 | P a g e 4. Duration of track clearance interval .......................................................24 Initiation of Preemption hold interval ...........................................................................31 Return to normal operations ........................................................................................32 What are the features of the systems? .............................................................................................38 Fail-Safe Design ...................................................................................................................38 Successive Operations..........................................................................................................39 Extended Advance Warning Times with median treatments .................................................39 Diagonal Railroad Crossing Both Highway Approaches to the intersection ............................39 Where is it implemented?................................................................................................................40 What are the results from implementations?....................................................................................42 Changes to Access Management ..........................................................................................42 Changes in Signal Timing .....................................................................................................42 Adding Turn Lanes ...............................................................................................................42 Queue Detection .................................................................................................................42 Changes to Operations at Downstream Intersections ...........................................................42 Signal Preemption ...............................................................................................................42 Police Enforcement..............................................................................................................43 What are the shortcomings and limitations? ....................................................................................44 Your ideas/suggestions for improving it (them) ................................................................................46 Conclusions and recommendations .................................................................................................47 References ......................................................................................................................................48
  • 4. Name: Kiomars Nassiri K. CEE 517 (Traffic Signal Systems) Term Paper UIN: 674851811 Interconnected Signals at Railroad Crossings 3 | P a g e Table of Tables and Figures Figure 1- Highway-rail grade crossing with nearby signalized intersection …..…………………………………..…6 Figure 2- Different types of issues with highway intersections in close vicinity of grade crossings……....8 Figure 3- Direct Current (DC) Train detection system method…………………………………………………………...13 Figure 4-Distribution of warning times by CWT systems…………………………………………………………….…...…15 Figure 5- Flashing light signal at grade crossing………………………………………………………………………………….16 Figure 6- Train demand and crossing operating indications periods……………………………………………..……20 Figure 7- Preemption Sequence for a two-phase traffic signal……………………………………………………………21 Figure 5- CAUTION—WALK TIME SHORTENED WHEN TRAIN APPROACHES sign………………………………..22 Figure 6- rail alignment crossing over two intersection approaches……………………………………………..……23 Figure 10 - Minimum track clearance distance (schematic diagram)………………………………………………….24 Figure 7 – Example Signal Preemption timeline………………………………………………………………………….……27 Table 1- Track Clearance Green Time for a Clearance Distance ‘L’ (Passenger Cars Only)…………………..28 Table 2- Track Clearance Green Time for a Clearance Distance ‘L’ (Passenger Cars + One Truck)……….28 Table 3-Track Clearance Green Time for a Clearance Distance ‘L’ (Passenger Cars + Two Trucks)………29 Figure 8- Train demand, crossing operating and train demand response time………………………………...29 Figure 9- train demand, crossing operating and traffic light response indications periods……………….30 Figure 12- Clearance phase……………………………………………………………………………………………………………….30 Figure 15- Changeable Message Sign and Head Signals to Restrict Turning Movements……………….……32 Figure 10- Time sequence of a grade crossing operation with preempted nearby highway crossing….33 Figure 11- Railroad crossing through the middle of a normal signalized intersection-Normal and Preemption Phase plan…………………………………………………………………………………………………………………….34 Figure 12- Skewed Railroad Crossing two approaches near a signalized intersection-Normal and Preemption Phase plan…………………………………………………………………………………………………………….……….35 Figure 13 – Railroad crossing on a two–lane roadway near a signalized intersection-Normal and Preemption phase plan……………………………………………………………………………………………………………………..36
  • 5. Name: Kiomars Nassiri K. CEE 517 (Traffic Signal Systems) Term Paper UIN: 674851811 Interconnected Signals at Railroad Crossings 4 | P a g e Figure 14- Railroad crossing on a two-lane roadway near a signalized intersection- Normal and Preemption phase plan…………………………………………………………………………………………….………………………37 Figure 15- Example of Observed Maximum Queues and Train Crossings (0700 – 1000) at Intersection with Queues Regularly Extending Past Railway Tracks………………………………………………………………………41 Figure 18- Sample Video Footage………………………………………………………………………………………………………41
  • 6. Name: Kiomars Nassiri K. CEE 517 (Traffic Signal Systems) Term Paper UIN: 674851811 Interconnected Signals at Railroad Crossings 5 | P a g e Introduction It is known that even under best circumstances, the railroad is an unforgiving environment and as a result grade crossings per se are associated with hazards and risks due to the limitations in train’s braking system, its high momentum, etc. As a result of that, the right-of-way is always given to trains at grade crossings. This might be a point of confusion for drivers since in the case of roadway signals, ROW is assigned alternately based on time or actuation [1]. Statistics show that fatalities and injuries as a result of grade crossing accidents are not that much correlated with the population. According to a United Nations report (2000), “more people die in grade crossing accidents each year in the United States than die in level crossing accidents in India.” The reason behind this number is the large number of crossing in the US and US’s higher level of motorization. Considering this fact, it’s incumbent on us to find engineering solutions for grade crossing-related issues. [19] In the case of a nearby highway intersection and so as to maintain ROW of trains, a good practice is to interconnect highway signal system to the active warning devices at grade crossing. In this fashion, chance of entrapment of vehicles on tracks-as a result of propagated queues from downstream intersection-would decrease. There’s an old saying in railroad industry that “rule books are written in blood” and safety regulations related to grade crossings in the close vicinity of highway junctions are not an exception at all. In this case, dramatic collision of a commuter train with a school bus in Fox River Grove, near Chicago, Illinois, with seven fatalities on October 1995 functioned as an eye-opener. The accident drew attentions towards the methods of treating this common geometrical configuration. The first report published on this issue after the accident on June 1997, described highway signal preemption as a “long-standing and desirable” engineering practice to prevent such accidents. There are two potential issues with the close proximity of highway signals to grade crossings. The first one which is more of importance is the queue spillback from the intersection to the grade crossing. This is going to be problematic when a train is arriving at the crossing and there are some vehicles entrapped on the tracks, taking away the ROW of the train. The other issue is the propagation of queues in the other direction from the crossing to the intersection which may result in severe operational conflicts at the intersection. As mentioned before, a common measure taken to
  • 7. Name: Kiomars Nassiri K. CEE 517 (Traffic Signal Systems) Term Paper UIN: 674851811 Interconnected Signals at Railroad Crossings 6 | P a g e mitigate safety issues as well as operational efficiency of the whole system is to coordinate railroad warning devices with intersection traffic signals. This special type of coordination is known as “Interconnection”. By interconnecting these signals together and defining a special operational mode for queue clearance in highway signal controller, one can clear vehicular queues on that segment of the road upon receiving an indication of the close proximity of trains to the crossing. So as Marshal defines, “The objective of a preempt is to take control of the nearby traffic signal to provide for the safe passage of a train, no matter what the status of the normal traffic signal operation at the time the preemption occurs.”[2] Figure 1 shows the focus area of this paper. Figure 1- Highway-rail grade crossing with nearby signalized intersection [7] (Focus area of this paper)
  • 8. Name: Kiomars Nassiri K. CEE 517 (Traffic Signal Systems) Term Paper UIN: 674851811 Interconnected Signals at Railroad Crossings 7 | P a g e What are the objectives of the paper? The first and foremost goal of this paper is to give the reader an idea about the importance and momentousness of the risks associated with grade crossings in close proximity of highway intersections. Although ordinary people and even educated engineers have always heard about how unsafe grade crossings can be and have a relatively good perception of the threatening dangers of rolling stock, they have probably never thought about the potential threat from the side of nearby highway intersections. Thus, the author of this paper has tried his best to explain this issue thoroughly by looking at it from different aspects. The second objective in this term paper is to help the reader understand advantages and disadvantages of state-of-the-practice preemption and interconnection methods. This is done by discussing features and elements of interconnecting highway traffic signal controller and the signal system at the grade crossing. Furthermore, logic behind preemption of signal controller at the adjacent intersection, train detection technologies, etc. is discussed. Another more important objective of this paper is to assess other possible measures that can be taken to tackle this problem and compare them with the focus area of this paper (interconnection of signals). In this manner, decision makers can get familiar with different options; choose the best available one based on their financial limitations, reliability of each measure and severity of their specific problem and finally get the optimal result.
  • 9. Name: Kiomars Nassiri K. CEE 517 (Traffic Signal Systems) Term Paper UIN: 674851811 Interconnected Signals at Railroad Crossings 8 | P a g e What are the important topics? 1. Main Issues with this configuration As mentioned before, there are two potential threats from the side of adjacent highway intersections when they are located in a short distance from railroad grade crossings. These two main issues are classified as Type 1 and Type 2 and are defined as follows: Type 1 (a.k.a “Influence Zone” Queue) is the queue of vehicles formed as a result of the existence of highway intersection signal. This queue has the potential to propagate upstream and in some cases up to the train tracks. Thus, if there’s no such preemption system in place, a train approaching the crossing would hit the vehicle fouling the tracks since there’s no room for the entrapped vehicle to clear train’s ROW. Type 2 (a.k.a “Gate Spillback” Queue) is specific to the cases that we have boom gates. In this case vehicles’ queue may extend from the crossing to the adjacent intersection, interfering with the operation of the intersection. (Figure 1) Figure 2- Different types of issues with highway intersections in close vicinity of grade crossings [3]
  • 10. Name: Kiomars Nassiri K. CEE 517 (Traffic Signal Systems) Term Paper UIN: 674851811 Interconnected Signals at Railroad Crossings 9 | P a g e There are different methods to alleviate problem type 1, some of which are as follows: 1. Using pre-signals: preventing queues forming across the level crossing by providing a controlling traffic signal on the approach to the level crossing. 2. Using queue detectors and actuation: minimizing queue length by installing a detector at grade crossing to identify queue formation. The detector actuates traffic signals quickly whenever queue reaches its location which results in dissipation of queued vehicles. 3. Preemption by Interconnection: discharging any formed queues when a train is approaching by providing a clearance phase which gives a green signal to queued vehicles. [main focus of the rest of this paper] Just like the previous type, there are different ways to prevent problem type 2 to happen. These are some practical ways: 1. Ensuring that vehicles (particularly long vehicles) will have sufficient time to clear the intersection and the level crossing before the level crossing starts operating. 2. Preventing vehicles from entering the roadway between the intersection and level crossing when the level crossing is operating. 2. When to Preempt? The turning point to a national attention to this issue was the Fox River Grove, Illinois, accident. In this accident that resulted in 7 fatalities, rear-end of a bus was on the tracks while the driver was waiting for the traffic signal to give him ROW to pass the intersection. Because of the absence of preemption at that location, bus driver couldn’t clear the tracks prior to the arrival of the train from the crossing. After the accident, there were lots of debates on the reasons behind the absence/malfunction of interconnection system at this specific location and other similar ones despite the fact that this system was in place in some other intersections. The conclusion was that up to the time of the accident, there had been no such accurate “guidelines existing as to when to provide interconnection of highway-railroad grade crossings with nearby traffic signals at intersections, relative to vehicle storage space between the intersection and the railroad crossing.”[1]
  • 11. Name: Kiomars Nassiri K. CEE 517 (Traffic Signal Systems) Term Paper UIN: 674851811 Interconnected Signals at Railroad Crossings 10 | P a g e As an aftermath of this deathly collision, Manual on Uniform Traffic Control Devices (MUTCD) is now giving some guidelines about the places that railroad engineers and traffic engineers should work together to interconnect these traffic signals. 3. Warning time Another important note about grade crossings is the amount of time needed for the preemption process to successfully clear the tracks before the arrival of the train on grade crossing. According to the Code of Federal Regulations, minimum amount of warning time to the roadway users should be 20 s before the train arrives at the intersection [8]. In order to provide drivers with this minimum time, we need to have train detectors placed far enough; at least this distance from the crossing should be equal to the distance traversed by the fastest moving train on that line to be on the safe side. It should be noted that this amount of time is mentioned as a minimum and whenever advance preemption is needed, engineers might come up with a larger amount of warning time. 4. Safety Issues with preemption system in place at intersections As it is going to be discussed in the next topic, traffic controllers that have preemption capabilities should have at least two sets of phase plans:  A normal set that the controller is permitted to run when there is no train demand.  A train demand phase set when there is a train demand. (Clearance mode) Now another important topic is the way controller makes a transition from normal mode to clearance mode. This might be challenging and can increase the risks to pedestrians and roadway traffic at the intersection who are not expecting their green phases to be abruptly terminated with no apparent cause at the time (Trains are typically out of sight at that moment). According to MUTCD, whenever preemption is needed, controllers can shorten pedestrian clearance phase but they’re not allowed to reduce time requirements of Yellow and All-Red intervals [4]. However, it’s not always the best practice to jeopardize pedestrian’s safety because of preemption. Whenever it’s inevitable, some other measures should be taken so as not to threaten lives of pedestrians. 5. Track clearance time The most important reason of adding a preemption system to the existing signal system is to make sure that the queues on tracks are cleared before the arrival of trains at crossings. By taking this
  • 12. Name: Kiomars Nassiri K. CEE 517 (Traffic Signal Systems) Term Paper UIN: 674851811 Interconnected Signals at Railroad Crossings 11 | P a g e objective into account, we should be able to estimate the required track clearance time. Constituents of track clearance time are Startup delay and Repositioning time and they’re going to be discussed later on. [5] 6. Installing Pre-signals Installing pre-signals are another safety layer on top of all the other measures taken to enhance safety on grade crossings. These signals are installed in front of railroad warning devices and are somehow coordinated with highway signals. The objective behind installing pre-signals is the same as interconnection and is nothing but preventing vehicles form queuing across the grade crossing. [6] These signals should display red while the grade crossing warning devices are operating. [17]
  • 13. Name: Kiomars Nassiri K. CEE 517 (Traffic Signal Systems) Term Paper UIN: 674851811 Interconnected Signals at Railroad Crossings 12 | P a g e What is the method/technique/concept behind it (them)? Before starting, it seems helpful to provide the reader with a clear definition of the most important terms of this paper: preemption and interconnection. USDOT defines these two terms as follows: Preemption: Transfer of normal operation of traffic signals to a special control mode. Interconnection: Electrical connection between the railroad active warning system and the traffic signal controller assembly for the purpose of preemption. [9] Four important components of preemption system are: 1. Train Detection Systems; 2. Highway-rail grade crossing warning devices and systems; 3. Highway traffic signals near highway-rail grade crossings; and 4. Interconnection of highway-rail grade crossing warning/control systems and highway traffic signals These components are going to be discussed respectively. Train Detection Systems In order to be able to preempt traffic signals, we should be able to detect trains well before the time they reach crossings. Thus we need to know about existing train detection systems. Researchers have named five different track-based train detection systems that are in use throughout the country: 1. Direct current (DC) or alternating current (AC) track circuits 2. AC-DC track circuits 3. Audio Frequency Overlay (AFO) track circuits 4. Motion sensor-controlled track circuits1 5. Constant warning time-controlled track circuits also called grade crossing predictors1 [7]   1 Most Commonly used
  • 14. Name: Kiomars Nassiri K. CEE 517 (Traffic Signal Systems) Term Paper UIN: 674851811 Interconnected Signals at Railroad Crossings 13 | P a g e  Fixed Distance systems The first three systems are known as fixed distance systems. They are somehow modified versions of the conventional train circuit invented by Dr. Robinson in 19th century. The concept behind their system is to take advantage of existence of conductor steel rails and use them as a part of energized electric circuit. When trains are approaching crossings, they shunt these circuits, activating grade crossing warning systems. These fixed distance systems are used all over the train network so in order to construct a specific grade crossing circuit, we should use insulated joints. In railroad terminology, an insulated joint is the border of two adjacent train circuits. (Figure 3) Figure 3- Direct Current (DC) Train detection system method [7]
  • 15. Name: Kiomars Nassiri K. CEE 517 (Traffic Signal Systems) Term Paper UIN: 674851811 Interconnected Signals at Railroad Crossings 14 | P a g e In order to be able to give sufficient warning time to the crossing users, we should account for the fastest train operating on the track(s) so as to extend the track circuit to a distance that the fastest train would travel within that required warning time (at least 20 seconds and in case of advance preemption more than that). This distance can be calculated with the following simple equation: df=rf×MWT (Eq. 1) Where: df= approach circuit distance for the fastest train operating on the track rf= fastest allowable train speed for the track in question MWT= minimum warning time provided to crossing users [1] In railroad industry we have a vast range of speeds of rolling stock from 10 to more than 110 miles per hour, so the most significant and also the most obvious problem with this system is when slow- moving trains are going to use the crossing. In these cases, 20 seconds warning time would increase to sometimes more than a minute. By taking the time it takes for the train to completely pass the crossing into consideration, roadway users may suffer more than 4 to 5 minutes of delay which is absolutely not a desirable thing to happen.  Constant warning time-controlled track circuits (CWT) These systems which are also known as grade crossing predictors are somehow modified and more adaptive versions of fixed distance systems. The difference is that although they also used track circuits for train detection, they continuously and while train is moving, measure train’s speeds and predict the arrival time of the train at grade crossing based on that.
  • 16. Name: Kiomars Nassiri K. CEE 517 (Traffic Signal Systems) Term Paper UIN: 674851811 Interconnected Signals at Railroad Crossings 15 | P a g e Figure 4-Distribution of warning times by CWT systems [7]  Other non-track-based systems In the advent of advancements in technology in the new millennium, different methods of accurate keeping track of moving stuff have evolved from which we can mention GPS tracking system. By using these new systems, reliability of the whole detection system enhances and trains can be located while they’re moving more precisely. Highway-rail grade crossing warning devices and systems In order to be able to explain interconnection and preemption methods, first we need to get familiar with some definitions that are going to be used throughout the paper. Since this traffic-related problem is on the interface of highway and Rail, the first thing we need to know is railroad industry’s terminology. There are two types of grade crossings which are classified by their distinct warning systems. The most common type, which is called Passive grade crossing, is only equipped with signs and possibly pavement markings. These warning devices just indicate the existence of a railroad crossing and will not give any further information about the proximity of trains. The most important issue with these low-cost crossings is that decision making on whether to pass the crossing or not is open to the drivers’ discretion. The other type, Active grade crossing, has flashing lights and sometimes gates
  • 17. Name: Kiomars Nassiri K. CEE 517 (Traffic Signal Systems) Term Paper UIN: 674851811 Interconnected Signals at Railroad Crossings 16 | P a g e and/or bells. The flashing light is used to give drivers a notification of the proximity of a train to the crossing. They are resting in inactive state when there’s no train in close proximity of the crossing until train reaches a certain point [7]. It should be noted that preemption system and circuitry can only be installed when grade crossing is equipped with active warning devices. Since the focus of this paper is interconnection between highway traffic signals and active grade crossing warning devices, some types of these warning devices at grade crossings are going to be introduced:  Flashing Light Signals and Bells: Although there might be some nuances between different countries’ flashing light design, the most common practice is to put a light pole with two flashing red light that alternately blink during the warning time and the time train is passing through crossing. Previously incandescent lamps were used as flashing lights in these warning devices but in the recent years, LED lamps have taken over due to their better performance and durability. In some cases, bells are also incorporated into these systems. (Figure 5 ) Figure 5- Flashing light signal at grade crossing  Automatic Gates (Boom Barriers): Some active crossings are equipped with automatic gates alongside flashing lights to enhance the operation of the whole warning system and reduce the chance of violation. In some countries such as Australia, whenever interconnection is implemented, boom barriers should be in place at grade crossing [10]. In
  • 18. Name: Kiomars Nassiri K. CEE 517 (Traffic Signal Systems) Term Paper UIN: 674851811 Interconnected Signals at Railroad Crossings 17 | P a g e its basic form, it is comprised of a pivoted bar named gate arm and a drive mechanism which blocks vehicle access through the level crossing. Although it’s not solid enough to block vehicles’ route and vehicles can pass the crossing by breaking the arm, research has shown that these arms are very effective in reducing crossing violations. [11].  Four-quadrant gates are the most recent version of boom barriers for protecting a grade crossing. In this configuration, the same mechanism is applied for both directions and in both sides of the crossing. One of the major drawbacks of this system is the possibility of entrapment of vehicles between the gates. In order to address this issue, several ways have been proposed, the most common of which is delaying the downward motion of exit gates for 5-7 seconds. [7]  Other components: Other components of active grade crossings such as event recorders, signs, etc. are less important in this case and are not going to be discussed here. In order to give highway signals an indication that warning devices at grade crossing are operating, a signal name crossing operating is sent from these devices to traffic signal controller. This signal starts when crossing warning lights start flashing and ends when the warning lights stop flashing. [17] Highway traffic signals near highway-rail grade crossings Whenever there’s a chance for the queue of stopped vehicles waiting at an intersection traffic signal to extend to the adjacent highway-rail grade crossing, special features should be considered to be added to these traffic signals. This added feature, called signal preemption, would be made possible with signal interconnection. As a result of deploying this system, vehicles trapped on the grade crossing tracks would be given a chance to clear the track prior to the arrival of the nearby trains. As mentioned before, the main issue with intersections near grade crossings is the possibility of queue spillback to both junctions that will either compromise safety of grade crossing users or will interfere with the operation of highway intersection.  Requirements of Interconnection: There are two main requirements that each preemption system at intersections near grade crossings shall follow: i. To provide enough time for the entrapped cars to clear the tracks prior to the arrival of the next train at the crossing.
  • 19. Name: Kiomars Nassiri K. CEE 517 (Traffic Signal Systems) Term Paper UIN: 674851811 Interconnected Signals at Railroad Crossings 18 | P a g e ii. To prevent any other movement from intersection towards the grade crossing during the preemption time. [12]  When to preempt traffic signals: In the latest version of MUTCD published in 2009, it is mentioned that intersections within 200 ft. distance of grade crossings should be equipped with preemption systems. Intersections farther than 200 ft. from crossings still should be considered as potential threats to the safety of crossings and in order to evaluate the need for preempting those signals, there’s a need for a detailed queuing analysis at each location. [4] Although no specific engineering methodology is stated in the code, but there are suggestions for the factors that should be considered such as traffic volumes, number of lanes, saturation flow rates, traffic signal timing, vehicular arrival characteristics and vehicle type. [4] In Canada, road/railway grade crossings technical standards and inspection, testing and maintenance requirements (Section 15) specify that: “The operation of traffic signals on a road approach shall be preempted by a grade crossing warning system: a) Where there is less than 60 m between the stop line for the traffic signals and the rail nearest the road intersection; or b) If it has been determined that traffic queued for the traffic signals regularly encroach closer than 2.4 m to the rail nearest the road intersection.” [12]  Characteristics of different types of preempted signal controllers: As discussed in the class, two of the most common signal controllers used throughout the US are Model 170 and NEMA TS2. Model 170 has six preemption routines, two of which, RR1 and RR2, are assigned to highway-rail grade crossing preemption [7]. TS2 controller, also, has 6 different preemption inputs which are identified as Preempt 1 to Preempt 6. The built-in assumption in these controllers is that the first two preemption plans are used for grade crossings but this assumption can be changed by the programmer. It should be noted that train preemption, whatever the number is, has priority over all other preemption routines.  Interconnection Circuitry: In order to preempt the highway signal controller, controller’s microprocessor should get informed of the approaching trains. In order to do so, train detection systems should be interconnected to highway signal controller’s processor via an isolated separate preemption circuit.
  • 20. Name: Kiomars Nassiri K. CEE 517 (Traffic Signal Systems) Term Paper UIN: 674851811 Interconnected Signals at Railroad Crossings 19 | P a g e  Preemption Phasing and Sequence: Korve (1999) proposes the following preemption sequence for all controller units:  Entry into preemption mode,  Termination of the current interval in operation (right-of-way transfer),  Initiation of “clear track” intervals,  Initiation of Preemption hold interval, and  Return to normal operations. In the following lines, the abovementioned operational sequence is going to be discussed:  Entry into preemption mode: In terms of reactions to preemption, there are two modes of operation namely locking and non-locking. Non-locking system is more sophisticated and accounts for the possibility of trains stopping after detection or changing their direction by waiting for a short time after train demand signal being sent to their controller. If the signal persists, they start to transition to the preemption mode. Locking systems, however, abruptly start shifting to preemption phasing upon receiving train demand signal. In order to enter preemption mode, a warning indication named train demand should be sent from the train detection devices to the highway signal controller. This signal starts when a train is detected approaching the level crossing and ends when the train has cleared the crossing. [17] The Train Demand has a true state (relay de-energized) from the time an approaching train is detected until the train has cleared the level crossing. [10]
  • 21. Name: Kiomars Nassiri K. CEE 517 (Traffic Signal Systems) Term Paper UIN: 674851811 Interconnected Signals at Railroad Crossings 20 | P a g e Figure 6- Train demand and crossing operating indications periods [17]  Termination of the current interval in operation (right-of-way transfer): Before shifting to the track clearance interval, an important issue is truncating current operating interval. In figure 6 this sequence is shown by letter P [7]. According to MUTCD rules, it’s not allowed to end the ongoing phase abruptly upon getting informed of nearby trains. The shortest time that each phase has to operate is comprised of minimum green, yellow change and red clearance time intervals. Although it is allowed by the laws to shorten the minimum green time, due to various safety concerns, the current practice is to have all these three time intervals for running phases. It’s worth mentioning that the reason behind not giving traffic engineers the authority to shorten yellow and all-red intervals is the possibility of entrapment of cars in the intersection.
  • 22. Name: Kiomars Nassiri K. CEE 517 (Traffic Signal Systems) Term Paper UIN: 674851811 Interconnected Signals at Railroad Crossings 21 | P a g e Figure 7- Preemption Sequence for a two-phase traffic signal [13] An important safety-related challenge associated with preemption is the fact that it can increase the risks to pedestrians and motorists who are not expecting their green phases to be abruptly truncated with no apparent cause at the time. Furthermore, evaluating adequate track clearance time precisely is not simple and is somehow be location-specific. In the recent years, state DOTs are shifting gear by paying more attention to pedestrian safety issues wherever interconnection and preemption is in place. In order to address this problem, different measure have been taken such as train-activated signage for pedestrians (Figure 8) and advance detection systems specifically for pedestrian clearance protection.
  • 23. Name: Kiomars Nassiri K. CEE 517 (Traffic Signal Systems) Term Paper UIN: 674851811 Interconnected Signals at Railroad Crossings 22 | P a g e Figure 8- CAUTION—WALK TIME SHORTENED WHEN TRAIN APPROACHES sign [14] Australian codes address this issue by allowing traffic signal controllers transition to the preemption special mode, provided that the following safety times are observed: o A minimum vehicle green of 5 seconds; o Pedestrian clearance times as set; o Yellow times as set; and o All red times as set. [17]
  • 24. Name: Kiomars Nassiri K. CEE 517 (Traffic Signal Systems) Term Paper UIN: 674851811 Interconnected Signals at Railroad Crossings 23 | P a g e Initiation of “clear track” intervals: The purpose of the clearance phase is to allow vehicles that may be trapped on the level crossing to clear the level crossing before or shortly after the warning lights start flashing and, where there are boom gates, before the boom gates start descending. [17] Three important points are important in this topic: 1. Number of track clearance intervals: Whenever rails cross over two different intersection approaches (Figure 9), there’s a need for more than one clearance interval so as to clear vehicles from both approaches separately and safely. Figure 9- rail alignment crossing over two intersection approaches 2. Track clearance signal indications displayed: With the ability of today’s traffic signals, traffic engineers can program signal controllers to display as many proper indications as needed whenever they have shifted to track clearance mode. This can incorporate right and left turn movements and has to include through movement for the vehicles clearing the grade crossing. 3. Clearance Configuration: In order to standardize the clearance storage distance and other clearance configurations, MUTCD in its 8th chapter defines minimum track clearance as “the length along a highway at one or more railroad tracks, measured either from the highway stop line, warning device, or 3.7 m (12 ft.) perpendicular to the track centerline, to 1.8 m (6 ft.) beyond the track(s) measured perpendicular to the far rail, along the centerline or edge line of the highway, as appropriate, to obtain the longer
  • 25. Name: Kiomars Nassiri K. CEE 517 (Traffic Signal Systems) Term Paper UIN: 674851811 Interconnected Signals at Railroad Crossings 24 | P a g e distance.” The following schematic diagram illustrates the above definition and makes it less vague. The minimum track clearance distance as described before should be totally clear of any vehicles. That is, any vehicle stopping at the grade crossing and blocking even a small portion of this distance, should be relocated to another position completely beyond this segment. Considering figure 10, front-end of vehicle A is within the predefined track clearance distance and as a result it should be repositioned whenever a train is approaching the crossing to a safer position, like vehicle B’s position. Figure 10 - Minimum track clearance distance (schematic diagram) [15] 4. Duration of track clearance interval: The most important of all is determination of the adequate operating time for queue clearance mode. According to MUTCD, operation time for the grade crossing flashing lights is 20 seconds before arrival of the train. Now the question is that if the same interconnection system is in place for traffic light preemption, would that 20 seconds be enough for clearing the queue or not? The common practice is that in formulation of the queue clearance time traffic engineers should account for the following aspects: • Grade crossing user vehicle types, e.g. cars, vans, single-unit trucks, etc;
  • 26. Name: Kiomars Nassiri K. CEE 517 (Traffic Signal Systems) Term Paper UIN: 674851811 Interconnected Signals at Railroad Crossings 25 | P a g e • Distance between the grade crossing and the intersection; and • Geometry/layout of the intersection, e.g. pavement type, horizontal alignment, vertical grades. [17] Among all the other efforts to quantify required clearance time, Gary Long has done an extensive analytical research on this topic. He believes that “While 20 sec might be adequate for railroad tracks that are within 200 ft. of signalized roadway intersections, it is usually grossly inadequate for separations approaching 500 ft.” [15] Although this concern has been addressed in MUTCD by allowing an increase in the 20- second warning time, it’s open to interpretation of traffic engineers since there’s no specific and objective guideline on the extent of this change. Practically speaking though, adding to the 20-second minimum warning time introduces some other safety issues to the whole system. In this way, motorists’ delay may become excessive tempting them to think of some risky behaviors in order to pass the crossing whenever it’s not safe to do so. Thus, in order to properly alleviate this problem, different types and arrangements of train detection devices will be required. There are different definitions for clearance time of the vehicles in queue when a train is detected to be approaching the crossing. A common practice is to split this clearance time into two separate components: a. Startup delay: It begins at the instant when a signal turns green allowing queued vehicles to move, and ends at the instant when the final vehicle at risk in a queue initiates movement. [15]  Since vehicles queueing at an intersection crossing will begin moving one after the other, startup delay is defined so as to account for this sequential movement.  The more vehicles are in the queue, the more startup delay the whole system will suffer from. So what matters here is more the number of vehicles in queue rather than the queuing distance.  Long proposes the following equation for this delay component:
  • 27. Name: Kiomars Nassiri K. CEE 517 (Traffic Signal Systems) Term Paper UIN: 674851811 Interconnected Signals at Railroad Crossings 26 | P a g e b. Repositioning time: It involves the time needed for the last vehicle to accelerate from rest and travel a distance sufficient to clear the crossing. [15] Since different types of vehicles have different acceleration methods, it’s not possible to derive a uniform formula for all vehicle types. Long (2002) has classified these formuli into three categories. The first case is applicable for roadways that only passenger cars are allowed to use. In the second case, roadways users include passenger cars, buses and single-unit trucks (combination trucks are not allowed). The last category includes all vehicle types. Further information about the ways these times are calculated can be found in reference [15]. Figure 11 better illustrates the sequence of events, by showing a generalized schematic timeline of both railroad and preemption events.
  • 28. Name: Kiomars Nassiri K. CEE 517 (Traffic Signal Systems) Term Paper UIN: 674851811 Interconnected Signals at Railroad Crossings 27 | P a g e Figure 12 – Example Signal Preemption timeline [1] Following tables are given based on the methodology used by Long and are presenting the total track clearance green time required to clear a queue with length ‘L’. A detailed analysis of these tables is available in [1].
  • 29. Name: Kiomars Nassiri K. CEE 517 (Traffic Signal Systems) Term Paper UIN: 674851811 Interconnected Signals at Railroad Crossings 28 | P a g e Table 1- Track Clearance Green Time for a Clearance Distance ‘L’ (Passenger Cars Only) [1] Table 2- Track Clearance Green Time for a Clearance Distance ‘L’ (Passenger Cars + One Truck) [1]
  • 30. Name: Kiomars Nassiri K. CEE 517 (Traffic Signal Systems) Term Paper UIN: 674851811 Interconnected Signals at Railroad Crossings 29 | P a g e Table 3-Track Clearance Green Time for a Clearance Distance ‘L’ (Passenger Cars + Two Trucks) [1] To ensure that the traffic signals are able to respond to the train demand and put the traffic signals in a state which is satisfactory for grade crossing operation (warning devices operating time), the train detection system must provide the train demand at a time in advance of the train arriving at the grade crossing. This time is the train demand response time. In Australia road authorities are responsible for providing the traffic engineers with the train demand response time required. [18] Figure 13- Train demand, crossing operating and train demand response time [17]
  • 31. Name: Kiomars Nassiri K. CEE 517 (Traffic Signal Systems) Term Paper UIN: 674851811 Interconnected Signals at Railroad Crossings 30 | P a g e Another indication that will result in efficient operation of the whole system is named traffic light response (TLR). This signal is sent from highway traffic signal to grade crossing warning devices when they’re in a state which is satisfactory for the level crossing to commence operation. This indication supposedly reduces the time that warning devices need to wait before starting their operation (train demand response time). If the TLR is not received the crossing will operate after the Train Demand Response Time has elapsed. Figure 14- train demand, crossing operating and traffic light response indications periods The clearance phase may be optionally extended beyond the minimum queue clearance time to clear all vehicles queued between the level crossing and the intersection stop line. Figure 15- Clearance phase
  • 32. Name: Kiomars Nassiri K. CEE 517 (Traffic Signal Systems) Term Paper UIN: 674851811 Interconnected Signals at Railroad Crossings 31 | P a g e “The level crossing boom mechanism Gate Delay is the time delay from the start of operation of the level crossing flashing lights and bells to the boom gates commencing to descend. The gate delay will be typically 12 seconds where ever the level crossing interfaces to the traffic signals.” [10] Whenever there’s a chance for reformation of queues, the Clearance Phase extends beyond the start of the level crossing flashing warning signals and concludes after the booms start to descend.  Initiation of Preemption hold interval Intervals occurring after track clearance interval in traffic signal controller while preemption is still in place are called Preemption Hold Intervals. During these intervals, traffic controllers give Right-of-way to some other conflicting movements that do not conflict with the train movement through grade crossing. Moreover, pedestrian movement directions not conflicting with the train movement are served. There are different modes of operation for preemption hold intervals throughout the world, some of which are going to be discussed below: 1. All Red: during preemption hold interval with this mode of operation, all the other intersection traffic lights will show red until train passes through the crossing. This operating mode obviously decreases the throughput of the whole intersection but in some cases such as places that railroad alignment is such that it crosses multiple intersection approaches (figure 9), this is the best practice. The reason is that there may be no other safe movement direction for vehicle’s at the intersection. 2. Flashing Red: works as if an all-way STOP sign is in place. In order to comply with the rules, intersection users should come to a complete stop at stop line before proceeding. This mode inhibits movements towards the railroad crossing. 3. Flashing Red/Flashing Yellow: Rules that should be abided by upon encountering these junctions are the same as the previous one; the only difference is that vehicles on the roadway parallel to tracks get flashing yellow sign. Thus, they do not need to come to a complete stop at the intersection and can cautiously move forward through the intersection. In some cases, in order to give drivers a better understanding of the situation, some changeable signs may be utilized. (Figure 15)
  • 33. Name: Kiomars Nassiri K. CEE 517 (Traffic Signal Systems) Term Paper UIN: 674851811 Interconnected Signals at Railroad Crossings 32 | P a g e Figure 16- Changeable Message Sign and Head Signals to Restrict Turning Movements [1]  Return to normal operations There are two different practices in servicing other movements after preemption phase is passed. The first one is first returning to those traffic movements that were most delayed as a result of preemption. The other one is first servicing those movements that have a potential for queue spillback to other intersections downstream. The second one might be problematic in some intersections because it may impose excessive delays to the other movement direction that is delayed for a long time before preemption and after that. Figure 16 recaps whatever discussed previously.
  • 34. Name: Kiomars Nassiri K. CEE 517 (Traffic Signal Systems) Term Paper UIN: 674851811 Interconnected Signals at Railroad Crossings 33 | P a g e Figure 17- Time sequence of a grade crossing operation with preempted nearby highway crossing Reference [1] has some recommendations on how to design signal preemption plans of different configurations of railroad alignments and nearby intersections. Following figures are taken from the same reference.
  • 35. Name: Kiomars Nassiri K. CEE 517 (Traffic Signal Systems) Term Paper UIN: 674851811 Interconnected Signals at Railroad Crossings 34 | P a g e Figure 18- Railroad crossing through the middle of a normal signalized intersection-Normal and Preemption Phase plan [1]
  • 36. Name: Kiomars Nassiri K. CEE 517 (Traffic Signal Systems) Term Paper UIN: 674851811 Interconnected Signals at Railroad Crossings 35 | P a g e Figure 19- Skewed Railroad Crossing two approaches near a signalized intersection-Normal and Preemption Phase plan [1]
  • 37. Name: Kiomars Nassiri K. CEE 517 (Traffic Signal Systems) Term Paper UIN: 674851811 Interconnected Signals at Railroad Crossings 36 | P a g e Figure 20 – Railroad crossing on a two–lane roadway near a signalized intersection-Normal and Preemption phase plan [1]
  • 38. Name: Kiomars Nassiri K. CEE 517 (Traffic Signal Systems) Term Paper UIN: 674851811 Interconnected Signals at Railroad Crossings 37 | P a g e Figure 21- Railroad crossing on a two-lane roadway near a signalized intersection- Normal and Preemption phase plan [1]
  • 39. Name: Kiomars Nassiri K. CEE 517 (Traffic Signal Systems) Term Paper UIN: 674851811 Interconnected Signals at Railroad Crossings 38 | P a g e What are the features of the systems?  Fail-Safe Design: Chris Wullems defines this concept as: “a design philosophy applied to safety–critical systems, such that the result of a failure either prohibits the system from assuming or maintaining an unsafe state, or causes the system to assume a state known to be safe.” [16] In order to maintain that conditions, track circuits used for train detection and interconnection circuits used for signal preemption should be energized when there’s no train interfering their circuit. The approach of the train, though, should de-energize these tracks to activate preemption mode. In this manner, whenever there’s a power failure in the whole system, signal will be preempted. Australian approach to fault monitoring and management is connecting traffic signal controller to SCATS to ensure that the site is monitored and alarms notified. In this case, if the controller finds a fault in any of the system components, it emits signals a high priority alarm and communicates it through SCATS. The common practice in this part of the world is that both railroad and roadway authorities should “nominate a contact person for each crossing and establish an agreed protocol for reporting problems with the operation of either system.” [17]  Successive Operations: Whenever there’s a possibility of trains arriving at grade crossing with short intervals, special measures need to be considered. Three of the general cases are discussed below: 1. The second train places a train demand while the first train demand is still in place. In highway interconnection terminology in this stage traffic controller is blind and will not “see” the new demand and the only thing it sees is the continuous demand. In this case, traffic controller will remain in the special preemption mode until the demand is ended. 2. In this case the second train’s demand is sent to the controller a few seconds after grade crossing warning devices finished their operation. In this case, the new demand is considered as a new demand and signal controller should go through all the preemption steps. 3. The last special case is when the second demand is sent to controllers and warning devices when the previous demand is ceased but warning devices are still operating. Now two different scenarios apply:
  • 40. Name: Kiomars Nassiri K. CEE 517 (Traffic Signal Systems) Term Paper UIN: 674851811 Interconnected Signals at Railroad Crossings 39 | P a g e a. The demand is received before the time that traffic signal controller is going to remove its traffic light response. In this case, the the traffic signal controller in the train demand phase set and remains here until “both” train demands have ceased. b. The second train demand is received after cession of traffic light response. In this stage the successive demand is treated as a new separate demand. [17] To ensure correct operation of the grade crossing/traffic signal interconnection, it is a requirement that the removal of the Train Demand is to be received by the traffic controller prior to the ending of grade crossing’s operation. This is achieved by the grade crossing continuing to operate until the Traffic Light Response is removed [10].  Extended Advance Warning Times with median treatments: In cases that grade crossings are equipped with boom gates and there’s a possibility of extending warning time to more than 45 seconds, it’s a common practice to include median treatments (such as adding a median) so as to prevent drivers bypass the gates.  Diagonal Railroad Crossing Both Highway Approaches to the intersection: whenever conditions of figure 9 exist, the common practice is to clear out traffic on both roadways prior to the arrival of the train which requires approximately twice the preemption time computed for one approach.
  • 41. Name: Kiomars Nassiri K. CEE 517 (Traffic Signal Systems) Term Paper UIN: 674851811 Interconnected Signals at Railroad Crossings 40 | P a g e Where is it implemented? Preempting highway signals when they’re in close proximity of grade crossings is one of the most common solutions to the potential threat of queue accumulation between the two junctions. Throughout the world, different countries such as Australia, Canada, United States, New Zealand, etc. have taken this measure to mitigate this problem. An extensive study on this issue is discussed next in which interconnection and some other alternative solutions were implemented in Canada. (Reference [20]) This study has been done in Region of York, Ontario, Canada on queues formed at 6 signalized intersections that appeared to be regularly extending from the traffic signals past a nearby set of railroad tracks. According to RTD-10 (Canadian code of operation-Transport Canada), all of these intersections should be equipped with signal preemption. This study was conducted so as to weigh the usage of signal preemption against other feasible solutions for mitigating queues at these junctions. It is mentioned at the report that “the purpose of this project is to present a methodology for analyzing and characterizing queues at signalized intersections in addition to identifying techniques for evaluating the effectiveness of potential mitigating solutions.” [20] The first stage in this study was to gather traffic information from these intersections. In order to do so, they used video trailers to record footage of traffic queued at intersections during the morning and afternoon peak hours which are 5-10 and 15-19 respectively. By reviewing these video footages, they collected some pieces of statistical information about some issues such as:  The queue lengths at the end of each signal cycle;  Traffic patterns contributing to the length of queues;  Time required for a vehicle at the end of a queue to proceed forward;  Frequency of train crossing the location and associated timing of events leading to and after the passage of the train; and  Frequency of train crossing the location and associated timing of events leading to and after the passage of the train.
  • 42. Name: Kiomars Nassiri K. CEE 517 (Traffic Signal Systems) Term Paper UIN: 674851811 Interconnected Signals at Railroad Crossings 41 | P a g e Then using SYNCHRO 7, traffic engineers did a comprehensive queue analysis on the observed data. In their queue modelling, the 95th percentile queue length was determined based on signal timing data and turning movement counts. Alternative solutions such as changes to signal timing, modifications to lane storage and addition of a lane, were assessed using the same software. These methods were only considered when LOS was more favorable than F. Figure 22- Example of Observed Maximum Queues and Train Crossings (0700 – 1000) at Intersection with Queues Regularly Extending Past Railway Tracks [20] Figure 23- Sample Video Footage
  • 43. Name: Kiomars Nassiri K. CEE 517 (Traffic Signal Systems) Term Paper UIN: 674851811 Interconnected Signals at Railroad Crossings 42 | P a g e What are the results from implementations? 7 different solutions to the aforesaid problem were found to be applicable to the 6 intersections of interest. These resolutions are reviewed here: 1. Changes to Access Management: in some cases there were some access routes just upstream of the railroad tracks which were one of the potential sources of traffic obstruction at the crossing. In order to address this issue, turn or time-based restrictions were imposed to the access routes. (An alternative for signal preemption that can be implemented in a short time frame) 2. Changes in Signal Timing: whenever LOS of the intersection was acceptable, adding to the green time to the railroad grade crossing approach was considered. In one case, SYNCHRO’s analysis showed a modest decrease in 95th percentile queue length. Also in another case, left-turning vehicles were source of queue spillback. For this case, when the protected left-turn phase was doubled, 95th percentile queue length decreased nosedived. (An alternative for signal preemption that can be implemented in a short time frame) 3. Adding Turn Lanes: in another intersection, review of video footage was showing a high volume of right-turning vehicles queueing at the intersection whereas there’s no separate right-turn lane. Engineers added a hypothetical exclusive right-turn lane to the geometry of intersection and observed an immense reduction in queue length. (A more costly alternative to signal preemption) 4. Queue Detection: another feasible solution was putting advance queue detectors near railroad grade crossing that could actuate traffic signal whenever a queue was forming near crossing. (A more costly alternative to signal preemption) 5. Changes to Operations at Downstream Intersections: At two intersections, traffic from another signalized intersection further downstream was queuing back to the traffic signals at the study intersection. As a result, Motorists couldn’t move through the intersection during the green phase, creating queues extending far beyond the railway tracks. A resolution to this problem might be changes to the signal timing or geometry of downstream intersections. (A more costly alternative to signal preemption) 6. Signal Preemption: in order to assess feasibility of signal preemption at these junctions and determine additional time requirements for queue clearance before arrival of train, TxDOT worksheet (prepared by the Texas Department of Transportation in the Guide for Determining Time Requirements for Traffic Signal Preemption at Highway-Rail Grade
  • 44. Name: Kiomars Nassiri K. CEE 517 (Traffic Signal Systems) Term Paper UIN: 674851811 Interconnected Signals at Railroad Crossings 43 | P a g e Crossings) was used. The conclusion was that only for one of the intersections, the required additional time was reasonable (22 s) while for the other 5 intersections the additional time was excessively long and impractical. (34-60s). The reasons that were mentioned in this report for infeasibility of additional time requirements of those 5 intersections are as follows: a. The greater distance between the signals and the railway tracks b. the greater uncertainty that the queue would be completely cleared prior to the arrival of the train; and c. The distance upstream that the advance warning would need to be placed to activate pre-emption at the traffic signals. 7. Police Enforcement: in intersections that video footage has shown numerous gate down violations and unsafe driving behavior, periodic enforcement of the law by police would be an option for a short-term period.
  • 45. Name: Kiomars Nassiri K. CEE 517 (Traffic Signal Systems) Term Paper UIN: 674851811 Interconnected Signals at Railroad Crossings 44 | P a g e What are the shortcomings and limitations?  It is a fact that in almost all of the cases, first stage of preemption, queue clearance phase, is wasted because rarely a crossing user stops on crossing no matter a train is approaching or not. Albeit this fact shows the inefficiency of train preemption system, this driving behavior seems to be adding a layer of safety to the whole process and in the first look, it might be considered to be a positive move from safety perspective; however, even that’s not the case. The problem is with the dynamic learning process of roadway commuters. After a while, drivers will learn that they would suffer less delay if they take advantage of the queue clearance mode. So whenever a train is approaching and warning devices are operating, motorists may choose to drive recklessly so as to pass the crossing and enjoy the reduced delay. Moreover, the amount of incurred delay by the first stage rises whenever the distance between two junctions increases.  As it was mentioned before, interconnection is a viable method whenever we have active warning devices in place at grade crossings. However, majority of grade crossing in the US are passive crossings that have no such a capability to be interconnected to the adjacent intersection in a short time frame. As a result, this solution is not feasible for a large number of grade crossings. In these cases, other alternative measures to signal preemption discussed before can be evaluated and implemented. Moreover, in cases that we have nearby un- signalized intersections with STOP and YIELD signs, interconnection cannot be used.  Fail-safe design of the whole interconnection system which seems to be the best practice is addressing safety issues in case of failure, might inadvertently add some societal risks. There are some drivers that lose their trust on warning devices as a result of experiencing undesirable long false warnings at grade crossings on and on. It might be as a result of failure in one of the components that changes the state of warning devices to their most restrictive mode of operation. Thus, drivers might disregard these warning devices and drive through crossings. There are some other small issues with the current systems that my result in malfunction of the whole system from which I can mention the following:
  • 46. Name: Kiomars Nassiri K. CEE 517 (Traffic Signal Systems) Term Paper UIN: 674851811 Interconnected Signals at Railroad Crossings 45 | P a g e Issues with interconnection circuitry: Train detection systems connect to traffic signals through some plug-in boards. Since there are lots of these plug-in boards inside these traffic signal controllers, a potential issue is the confusion in determining the appropriate interface location in case these plug-in boards are not labeled or are labeled inconsistently.
  • 47. Name: Kiomars Nassiri K. CEE 517 (Traffic Signal Systems) Term Paper UIN: 674851811 Interconnected Signals at Railroad Crossings 46 | P a g e Your ideas/suggestions for improving it (them) This method per se is not a guarantee for having 100% safe grade crossing. The most important contributing factor to accidents on grade crossings is the human factor. Preemption would not work as effective as we might think if drivers are not complying with the rules. That is, if motorists pass the flashing-lights or bypass the boom gates, preemption cannot be helpful even if it’s perfectly tuned. So besides working on engineering part of these geometric configurations, it’s incumbent on us to put some time and effort to teach the correct driving decisions to drivers. There are three major distinct devices involved in the process of preemption and interconnection: highway traffic signals, train detection systems, highway-rail grade crossing warning devices. The status quo is that each of these parts of the system are manufactured and designed separately. My recommendation is that since all of them are going to work together, it’s better to have them planned and manufactured altogether. In this manner, consistency in operation of these systems would be enhanced and maintaining fail-safe logic would become easier. Moreover, using newer train detection technologies such as GPS can make the whole system more reliable because it reduces failures as a result of mechanical and electrical circuits used in today’s train detection devices. Also, the Positive Train Control (PTC) system that is in the process of implementation on the entire railroad tracks throughout the country, at least at experimental stage, has shown great abilities in terms of accident prevention.1 1 For more information about this new system, take a look at the following article: "Positive Train Control." Wikipedia. Wikimedia Foundation, n.d. Web. 06 May 2015. <http://en.wikipedia.org/wiki/Positive_train_control>
  • 48. Name: Kiomars Nassiri K. CEE 517 (Traffic Signal Systems) Term Paper UIN: 674851811 Interconnected Signals at Railroad Crossings 47 | P a g e Conclusions and recommendations From the societal perspective, although stopping vehicles at the grade crossing and on the rails, regardless of the fact that train is approaching or not, is a careless and irresponsible behavior, it’s something that might happen once in a while and the serious damage and injury as a result of that is not an example of proportionality between crime and punishment at all. So it reasonable to invest on this traffic-related issue and try to enhance safety in this area. Whenever active warning devices at grade crossings are present, cost of adding preemption system and installing interconnected circuits is not prohibitive; however, there are some indirect costs such as added delay cost to intersection users that should be accounted for whenever putting these systems in place is considered for a specific location. In other words, we can conclude that equipping grade crossings with this advanced system in most of the cases is associated with a trade- off between the lives of reckless faulty drivers and imposed delay to innocent intersection users. Grade crossings and their adjacent highway intersections involve two completely separate organizations. Traditionally, rail and road signals at grade crossings have had separate installations, even when they have been in close proximity. However, it has been recognized that closer co- operation between the rail and road signals may decrease the potential for motor vehicles queuing across the level crossing. The difference in their operating practices is to that extent that even the terminology used by roadway authorities is in some cases different from railroad authorities. Thus, In order to maintain safety and enhance the overall operation of these systems, there should be a mutual understanding between involved parties. For achieving this goal, both parties should inform each other of the design concepts and changes in operation of their control systems whenever it’s effecting the interconnection and preemption systems. [10]
  • 49. Name: Kiomars Nassiri K. CEE 517 (Traffic Signal Systems) Term Paper UIN: 674851811 Interconnected Signals at Railroad Crossings 48 | P a g e References 1. Datta, Tapan K., Timothy J. Gates, Peter T. Savolainen, Ahmad Fawaz, and Amna Chaudhry. Timing Issues for Traffic Signals Interconnected with Highway‐Railroad Grade Crossings. Rep. no. RC-1578. Detroit: Wayne State University, February 2013. 2. Marshall, P. S. (1997). "An Investigation of Railroad Pre-emption at Signalized Intersections." Department of Civil Engineering, University of Wisconsin, Madison, May, 1989. 3. Suggett, Jeff, and Paul Nause. Analysing and Mitigating Queues at Signalized Intersections Adjacent to Railway Crossings. Rep. N.p.: Associated Engineering, 2013. 4. FHWA,. (2009). Manual on Uniform Traffic Control Devices for Streets and Highways, Part 8 and part 4, Traffic Control for Railroad and Light Rail Transit Grade Crossings. 5. Long, Gary. Clearance Time Requirements at Railroad-Preempted Traffic Signals. Rep. no. 4504609-12. Gainesville: U of Florida, 2002. 6. Gilleran, Brian F. "Use of Pre-Signals in Advance of a Highway- Rail Grade Crossing: A Specialized Tool with Specific Applications." ITE Journal (May 2006): 22-25. 7. Korve, H. W. (1999). "NCHRP Synthesis of Highway Practice 271: Traffic Signal Operations Near Highway-Rail Grade Crossings." TRB, National Research Council, Washington, D.C. 8. Code of Federal Regulations. Title 49, Part 234.225. Activation of Warning Systems. 9. USDOT Technical Working Group, “Implementation Report of the USDOT Grade Crossing Safety Task Force”, U.S. Department of Transportation, Washington D.C., June 1997. 10. Level Crossing – Traffic Signal Design Interface Agreement. Tech. 2nd ed. Sydney: RailCorp – Roads & Traffic Authority, 2010 11. Rudin-Brown, Christina M., Michael G. Lenné, Jessica Edquist, and Jordan Navarro. "Effectiveness of Traffic Light vs. Boom Barrier Controls at Road–rail Level Crossings: A Simulator Study."Accident Analysis & Prevention 45 (2012): 187-94. 12. Road/railway grade crossings technical standards and inspection, testing and maintenance requirements. Rep. no. RTD 10. N.p.: Rail Safety Directorate Safety and Security Transport Canada, 2002.
  • 50. Name: Kiomars Nassiri K. CEE 517 (Traffic Signal Systems) Term Paper UIN: 674851811 Interconnected Signals at Railroad Crossings 49 | P a g e 13. Traffic Engineering Council Committee 4M-35, Recommended Practice, Preemption of Traffic Signals At or Near Railroad Grade Crossings with Active Warning Devices, Institute of Transportation Engineers, Washington D.C., 1997. 14. Illinois Standard Highway Signs Book. N.p.: Illinois Department of Transportation, July 2014. 15. Long, Gary. Clearance Time Requirements at Railroad-Preempted Traffic Signals. Rep. no. 4504609-12. Gainesville: U of Florida, 2002. 16. Wullems, Chris. "Towards the Adoption of Low-cost Rail Level Crossing Warning Devices in Regional Areas of Australia: A Review of Current Technologies and Reliability Issues." Safety Science 49.8-9 (2011): 1059-073. 17. Australia. Roads and Traffic Authority (RTA). New South Wales. Traffic Signal Design (Appendix F-Level Crossing Interface Concept of Operation). 1st ed. N.p.: Roads and Traffic Authority, 2010. 18. Level Crossing Design. Rep. no. ESD-03-01. N.p.: Australian Rail Track Corporation, 2012. Print. Engineering (Signaling) 19. ECONOMIC AND SOCIAL COMMISSION FOR ASIA AND THE PACIFIC. Evaluation of Cost-effective Systems for Railway Level-crossing Protection. New York: United Nations, 2000. 20. Suggett, Jeff, and Paul Nause. Analysing and Mitigating Queues at Signalized Intersections Adjacent to Railway Crossings. Rep. N.p.: Associated Engineering, 2013.

Related Documents