Semisequicentennial Transportation Conference Proceedings
May 1996, Iowa State University, Ames, Iowa

Design of Gate Delay and Gate Interval Time for Four-Quadrant Gate System at Railroad-Highway Grade Crossings

Fred Coleman and Young J. Moon

Department of Civil Engineering,
University of Illinois at Urbana-Champaign,
205 N. Mathews Avenue,
Urbana, Ilinois 61801.

A design methodology for gate delay and gate interval time for at-grade crossings utilizing four-quadrant gates is developed. The design approach is based on the concept of dilemma zones related to signal change intervals at signalized intersections. The design approach is validated based on data from six sites in Illinois on a proposed High Speed Rail corridor. Gate delay and gate interval times are determined which provide an optimal safe decision point to allow a driver to stop before the crossing or proceed through the crossing without becoming trapped by the exit gates.


The planning for introduction of new high speed (HS) rail passenger train service under Section 1010 of the Intermodal Surface Transportation Efficiency Act of 1991 (ISTEA) in the Chicago-St. Louis corridor involves, among many other activities, the review of grade crossing protection. The U.S. Department of Transportation (DOT) has established guidelines for grade crossing protection based on ranges of operating speeds (1). The Illinois Department of Transportation (IDOT) is proposing to operate high speed rail passenger service in this corridor at speeds of 125 mph (2). At these speeds the DOT guidelines require grade separation or the demonstration of new technologies which absolutely preclude entry into the crossings. To achieve the latter, IDOT has under consideration Four Quadrant Gates (QG) in conjunction with a Trapped Vehicle Detection (TVD) System. Quad gate installations have recently been placed in operating service on light rail and freight lines (3) to take advantage of their inherent ability to eliminate crossing violations after gate arms have been lowered.

Quad gate operation implementation issues can be initially divided into two categories: (a) gate operations; meaning operation of the gates related to timing of lowering entry (near) and exit (far) arms to allow vehicles to clear, and (b) constant warning times (CWT); meaning how should constant warning times be incorporated at crossings with QG serving freight and HS passenger trains.

This paper uses the analogy of a dilemma zone from research on traffic signal change intervals which assure that a high percentage of drivers can either clear the intersection or stop before entering. A railroad crossing adaptation of this concept is suggested to characterize driver behavior in response to initiation of flashing lights and activation of the gate descent. Operating data from six sites under consideration for four quadrant gates in the Chicago-St. Louis corridor are utilized to determine: 1) gate delay, the time interval after initiation of flashing lights, and 2) gate interval time, the time interval between entry and exit gate descent to assure that vehicles will have sufficient time to clear the crossing.

The long-term goal is to determine the operating time parameters which insure a safe system operation with the integration of quad gates into existing crossing safety systems such as Constant Warning Time (CWT) signals.

QUAD GATE OPERATIONS

Function and Purpose

Quad gates are an additional pair of dual gate arms which are lowered on each side of a bi-directional crossing preventing any vehicle from crossing in-between the lowered gates because their travel path is blocked on the front and rear side of the crossing. The implementation of quad gates complicates the crossing scenario because the likelihood of a vehicle clearing the crossing prior to exit gate descent must be taken into consideration. A primary concern is the possibility of a vehicle becoming "trapped" between the entry and exit gates. In addition, other issues arise in the operation of quad gates:

1) Introduction of a second gate arm (exit) prohibiting travel in the same direction may create driver indecision leading to a trapped vehicle incident,

2) The distance between multiple tracks will have an impact on exit gate timing,

3) Track crossing roughness may be a critical factor in some situations, and

4) Tractor-trailer and hazardous material vehicles in any combination of 1), 2), or 3) present potential for a worst case scenario for vehicle crossings.

Figure 1 is an example of a four quadrant gate crossing. Included in this figure are the areas on the approach to the crossing which define roadway lengths where drivers are involved in the decision of whether to stop or to proceed through the crossing. The primary safety benefit from quad gates is that they assure no crossing violations after the gate arms are lowered, unless the gate arms are penetrated. The implementation of four quadrant gates, while eliminating gate arm violations, does present the potential for trapping a vehicle. Similar to a signalized at-grade intersection, vehicles approaching at various speeds must be allowed to clear the intersection during the yellow change interval. The at-grade crossing must therefore be analyzed relative to a change interval based on the ability of vehicles to clear the intersection. Driver behavior relative to stopping or proceeding at various speeds is the primary determinant of the likelihood of clearing the intersection.

Experience and Applications

Currently in the United States, there is limited experience with a QG system. Richards et al. (4) have obtained field trial results which involves crossings with freight train traffic. Their study sites had low to moderate vehicular volume with few trucks or vehicles required to stop before crossing. In the field trial, quad gates operating with flashing lights similar to standard gates, their effectiveness in reducing crossing violations after initiation of flashing lights was statistically significant at the 99 percent confidence level. For crossing violations after QG arms were down the effectiveness was statistically significant at the 99 percent confidence level.

Heathington et al. (5) identified six characteristics of crossings where quad gates [with skirts] would be good countermeasures to alleviate gate arm violations. These six characteristics are as follows:

(a) crossings with a large number of hazardous materials trucks or trains carrying hazardous materials,

(b) crossings with a large number of school buses,

(c) crossings with high speed trains.

Three of the six characteristics were present at the six sites analyzed in this research:

(a) crossings with a large number of hazardous materials trucks or trains carrying hazardous materials,

(b) crossings with a large number of school buses,

(c) crossings with high speed trains.

Gate Operations

The literature (3,5,6) on quad gates has made suggestions for gate operations, with specific design guidelines suggested in 1993 by the Federal Railroad Administration (7). Gate operations are composed of two components as follows:

If implementation of quad gates is to be undertaken, there needs to be design criteria which assure the safety of motorists based on the operation of quad gates and the likelihood of clearing the crossing or stopping in an appropriate manner.

Gate Delay

Gate delay is five seconds for dual gate crossings in Illinois. For quad gates no change in this operating policy is currently contemplated. The MUTCD (8) requires not less than 3 seconds for gate delay at grade crossings with dual gates after flashing signals commence. The Federal Railroad Administration (FRA) (7) suggested guidelines is three seconds based on their 1993 amendment to Emergency Order Number 15 with respect to application in Florida.

Gate Interval Time

In proposed guidelines issued in September 1993, the Federal Railroad Administration (7) has suggested that gate interval time (measured after initiation of entrance gate lowering) be 1 to 3 seconds. The New York State Department of Transportation (NYSDOT) in their High Speed Technology Demonstration Proposal (6) indicate an operating scenario which provides 10 to 12 seconds of gate interval time prior to initiation of descent of the exit gates.

Dilemma Zone Concept with Application to Railroad-Highway At-Grade Crossings

History and Application

The dilemma zone concept refers to the research and methodology pioneered in traffic engineering related to drivers decisions to stop or proceed at the onset of the yellow change interval. Sheffi and Mahmassani (9) state "The dilemma refers to the drivers' decision to proceed through the intersection or to stop when the signal indication changes from green to amber." The concept of a dilemma zone was recognized in the work of Gazis et al. (10), Olson and Rothery (11), Crawford (12), and Herman (13) and defined by Sheffi and Mahmassani (9) ''as that zone within which the driver could neither come to a stop nor proceed through the intersection before the end of the amber phase." Continued work on this concept has led to a probabilistic approach to a driver stopping, with Zegeer (14) defining a dilemma zone as "the road segment where more than 10 percent and less than 90 percent of the drivers would choose to stop." Sheffi and Mahmassani (9) indicate "The approach consists of developing dilemma zone curves of "percent drivers stopping" versus "distance from stop bar" at the instant when the signal indication changes from green to amber."

Basis for Dilemma Zones at Railroad-Highway At-Grade Crossings

Drivers approaching an at-grade railroad crossing are faced with a similar scenario in which a visual signal in the form of flashing lights informs the driver of the need to stop, before the descent of the gates. The similarity to a signalized intersection is that when drivers are some distance away from a crossing with gates when commencement of flashing lights occur, they must make a decision to stop or proceed. Therefore, determination of the zone boundaries for railroad-highway grade crossings based on "safe stopping distances" and the "clearance distance" given the approach speed and the width of the crossing using typical values of acceleration and deceleration rates are possible, similar to work performed by Gazis et al. (10).

Figure 2 shows the definition of dilemma and option zones which a driver faces when he/she is approaching an at-grade crossing.

Stopping Distance and Clearance Distance

In order to design the gate delay and gate interval time at railroad-highway grade crossings with quad gates, a dilemma zone is established in terms of the relationship between stopping distance (Xs) and clearance distance (Xc). A driver approaching an at-grade crossing during gate delay, the time that the flashing light signals are operated before the entry gates are activated, will either have to stop or proceed to clear the crossing. Figure 3 shows the QG system of a crossing which includes all geometric data for calculating stopping distance as well as clearance distance.

The stopping distance is the distance required for vehicles to stop at the stop bar usually 6-8 ft in front of the gate arm. This stopping distance for an at-grade crossing is the same for signalized intersections, except it includes the distance between stop bar and gates. It is formulated as follows:

[--- Pict Graphic Goes Here ---]

[1]

where,

Xs = stopping distance (ft);

t = driver perception-reaction time (PRT) (sec);

v = approach speed (ft/sec);

a = deceleration rate on level pavement (ft/sec2);

G = acceleration due to gravity (ft/sec2);

g = grade of approach lanes (percent/100); and

D = distance between stop bar and gates (ft).

The clearance distance is defined as the distance within which the driver can clear the crossing before the end of the quad gate operation time (i.e. gate delay plus gate interval time). Therefore, during the quad gate operation time the vehicle travels a total clearance distance which is: (1) the continuation distance, (2) the distance between entrance and exit gates, and (3) the length of the vehicle. Referring to Figure 3, considering the crossing angle between the railroad and highway, as well as road segments immediately adjacent to the railroad track, the continuation distance required for a vehicle to clear the crossing can be formulated as follows:

[2]

where,

Xcon = continuation distance (ft);

TG = gate operation time (sec);

TG . v = total clearance distance (ft);

Wt = width of railroad track (ft);

Wh = width of approaching lane of the highway (ft);

Wg = distance from track edge to gate (ft);

a = crossing angle (deg); and

L = length of the vehicle (ft).

As shown in Figure 2a, if a vehicle approaches a crossing during gate delay and if Xs > Xc and the vehicle is positioned between Xs and Xc such that Xs > X > Xc, a dilemma zone exists where a vehicle could neither stop nor clear the crossing. Referring to Figure 2b, if Xs < Xc and the vehicle is positioned between Xs and Xc such that Xs < X < Xc, an option zone exists for which a driver can choose between stopping and clearing the crossing. If Xs = Xc, the dilemma and option zones are eliminated and a point or distance is obtained which assures the likelihood of drivers stopping or clearing the crossing. This distance is called a "Safe Decision Location."

Design of Gate Delay and Gate Interval Time

Based on the "Safe Decision Location", where Xs = Xc, the simplification results as follows:

[3]

where,

TG = gate operation time (sec);

TD = gate delay (sec);

TI = gate interval time (sec).

The first term, the gate delay (TD) is independent of the crossing angle; however, the gate interval time (TI) must include the crossing angle. Gate delay is based on human factors and driver behavior. Using approach speeds, t (driver perception-reaction time, PRT) value of 1.0 sec and 2.5 sec, and varying the deceleration rate, Figure 4 presents the gate delay requirements. With a deceleration rate of 10 ft/sec2 similar to the intersection studies with an approach speed of 35 mph, 3.7 sec. of gate delay is required. As a whole, the gate delay should be approximately 3.0-4.0 sec at t (PRT) = 1 sec, or 4.5-5.5 sec at t (PRT) = 2.5 sec based on AASHTO (14). However, slightly longer times may be justified if vehicle approach speeds are over 40 mph.

Figure 5 shows the gate interval time requirements based on the second term in Equation 3 in terms of speed in the track zone. This gate interval time is the total gate interval time available. If the interval is assumed to begin at the start of entry gate descent, and a vehicle at the entry gate decides to clear the crossing only 2–3 seconds is available before the gate would come into contact with the vehicle. Utilization of this 2–3 seconds does not diminish the total gate interval time for this vehicle since the same amount of time to avoid contact with the exit gate is available.

In assessing the criteria for gate interval time, the ability of a vehicle to clear the railroad track zone under low speed conditions was felt to be the primary consideration from a safety standpoint. Figure 5 indicates that for autos, vehicle speed in the track zone (see Figure 1) is the primary factor in determining gate interval time. At speeds less than 10 mph, gate interval time increases sharply, while at speeds greater than 10 mph, approximately 5 seconds is sufficient. This finding indicates that crossing speed as determined by grade crossing roughness, and driver behavior such as looking, precautionary slowing, preceding vehicular clues or traffic lights should be recognized as one of the primary considerations in the determination of gate interval time. However, if the gate interval is excessive in length the credibility of the quad gate system to drivers will be lacking.

Concept Validation Based On Field Data

In order to determine both gate delay and gate interval time for actual sites, six crossings under consideration for four quadrant gates in Illinois are evaluated. Each sites operating and geometric data are shown in Table 1.

Table 2 shows the gate delay needs at the six sites utilizing a deceleration rate of 10 ft/sec2 . Gate delay with 1 second of PRT is approximately 4 seconds at an approach speed of 35 mph and 5 seconds with an approach speed of 45 mph. For 2.5 seconds of PRT, approximately 5 seconds of gate delay at 35 mph is required and 6 seconds at 45 mph. Gate delay is independent of the crossing angle and depends only on the approach speed and deceleration rate, as indicated in the first term of Equation 3.

Table 3 indicates the gate interval time needs at these sites. Five of the crossings are not at a right angle. In addition, the Gardner and Pontiac site crossing surfaces have been evaluated as "rough" (15), suggesting that minimum vehicle speeds are appropriate. All other sites evaluated as "good" have assumed crossing speeds of 5 mph. All the crossings are level (0 percent grade).

Utilizing the geometry and assumed minimum speed on the track zone, the overall gate interval time varies from approximately 7-15 seconds for a passenger vehicle. For a WB-60 truck (16) which has 65 ft. of vehicle length, approximately 14-22 seconds of gate interval time would be required for clearing the vehicle safely at the crossing. It should be noted that the major factors which influence the gate interval time are the minimum vehicle speed in the track zone, width of the crossing (i.e. distance between entrance and exit gate), the length of the vehicle, and the angle of the crossing.

Findings

Table 4 and Table 5 present the input values and results to determine the Safe Decision Location (Xs = Xc) distance using the approach speed and then the minimum speed in the track zone for automobiles.

The difference between the clearance distance and stopping distance is computed for both assumed speeds in the track zone. The comparison of the difference between Safe Decision Location using approach speed and minimum speed on the track zone determines if the speeds in this area require a different gate interval time. The results in the last column of Table 4 and Table 5 indicate that the differences in the Safe Decision Location are insignificant and therefore the gate interval times computed for the minimum speed in the track zone is adequate and does not provide excessive gate interval time for vehicles crossing the track at normal speed.

Conclusions

The approach suggested along with the findings presented utilizing site data from crossings in Illinois indicate that utilizing the concept of a dilemma zone provides a design procedure for gate delay and gate interval times. Further, it is demonstrated that by incorporating the geometry of a site, speed of approach, width of crossing, and minimum assumed speed in the track zone, a sufficient gate interval time for automobiles could be provided. This overall design approach was validated through comparison of the Safe Decision Location and was found to provide essentially the same distance for a driver to stop or continue through the crossing, since the gate operation time included the necessary gate interval time to allow clearance of the crossing.

REFERENCES

  1. The Highway-Rail Grade Crossing Safety Initiative. Federal Railroad Administration, U.S. Department of Transportation, December 1992.
  2. Chicago-St. Louis High Speed Rail Financial and Implementation Plan - Summary Report. Illinois Department of Transportation, 1994.
  3. L. Meadow. Light Rail Transit Safety Issues. In Transportation Research Board 73rd Annual Meeting, Washington, D.C., January 1994.
  4. S. H. Richards and K. W. Heathington. Assessment of Warning Time Needs at Railroad-Highway Grade Crossings with Active Traffic Control. In Transportation Research Record 1254, TRB, National Research Council, Washington, D.C., 1990, pp. 72–84.
  5. K. W. Heathington, S. H. Richards, and D. B. Fambro. Guidelines for the Use of Selected Active Traffic Control Devices at Railroad-Highway Grade Crossings. In Transportation Research Record 1254, TRB, National Research Council, Washington, D.C., 1990, pp. 50–59.
  6. New York State's High Speed Technology Demonstration Proposal. New York State Department of Transportation, October 1993.
  7. Emergency Order No. 15, Notice No. 4. Amendment of Emergency Order No. 15. Federal Register, Vol. 58, No. 177, September 15, 1993, pp. 48415–48420, Federal Railroad Administration.
  8. Manual on Uniform Traffic Control Devices (MUTCD). U. S. Department of Transportation, FHWA. Washington, D.C. , 1988
  9. Y. Sheffi, and H. Mahmassani. A Model of Driver Behavior at High-Speed Signalized Intersections. Transportation Science, Vol. 15, No. 1, February 1981, pp. 50–61.
  10. D. Gazis, R. Herman, and A. Maradudin. The Problem of the Amber Signal Light in Traffic Flow. Operations Research, Vol. 8, No. 1, 1960, pp. 112–132.
  11. P. L. Olson and R. W. Rothery. Deceleration Levels and Clearance Times Associated with the Amber Phase of Traffic Signals. Traffic Engineering. Vol. 42, No. 4, April 1972, pp. 16–19.
  12. A. Crawford. Driver Judgment and Error During the Amber Period at Traffic Lights. Ergonomics, Vol. 5, 1962, pp. 513–532.
  13. R. Herman et al. Problem of the Amber Signal Light. Traffic Engineering and Control, Vol. 5, 1963, pp. 298–304.
  14. C. Zegeer. Effectiveness of Green-Extension Systems at High-Speed Intersections. Kentucky Department of Transportation, Research Report 472, May 1977.
  15. National Railroad-Highway Crossing Inventory. Illinois Department of Transportation, 1994.
  16. A Policy on Geometric Design of Highways and Streets. AASHTO, Washington, D.C., 1990.

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