Active Treatments
Overview
Active traffic control devices are those that give visual and audible advance notice of the approach of a train. These include flashing ‑light signals (both mast ‑mounted and cantilevered), bells, automatic gates, active advance warning devices, and highway traffic signals. Active devices are typically activated by the passage of a train over a detection circuit in the track, except in those few situations where manual control or manual operation is used (LRT systems typically use loops or contacts because LRT tracks are used as an electrical “ground” to propel trains). Active traffic control devices are usually supplemented with the same signs and pavement markings used for passive control, except that STOP or YIELD signs are not used where active traffic control devices are installed. This section describes the different traffic control devices available for use. Additional guidance on the selection of alternatives can be found in Chapter 3 . Driving tasks at crossings with active traffic control devices are different from those at crossings with passive devices. Passive devices indicate that a crossing is present and that it is the road user’s responsibility to look for an approaching train and take appropriate action. At crossings with active traffic control devices, a motorist is told when a train is approaching or is present at the crossing. It is the motorist’s responsibility to take appropriate action when the devices are activated.
Train Detection and Device Activation
Devices need to be actuated in advance of train arrival to allow road users to clear the track area. A key principle is “warning time”—the amount of advance notice of the arrival of a train. In addition to the time for road users to perceive and react to active warning devices and for pedestrians and vehicles to clear the tracks, active control devices present at a crossing must activate and operate in a logical sequence which may include “preemption” of downstream traffic signals to allow vehicles to clear from the crossing, as well as activation of flashing ‑light devices and bells followed by descent of the crossing gates, if present. Active devices are actuated by means of train detection circuitry. Legacy train detection units rely upon conventional electronic components including coils, resistors, and relays. Current train detection is largely solid state driven, but the outputs are taken from relays activated by the solid state circuits. Shown in Figure 20, the fundamental concept is use of a railroad circuit which uses the rails as conductors. A voltage is applied across the rails at one end of a detection zone and a relay (or other circuitry) detects the voltage transmitted down the rails. The presence of a solid electrical path, as provided by the wheels and axles of a locomotive or railroad car, shunts (closes) the circuit, which causes the detection relay to trip. The system is also designed to be fail-safe; that is, any shunt of the circuit, whether by railroad equipment, vandalism, or an “open circuit” (e.g., a broken rail or track connection), will open the relay, which causes the crossing signals to be activated. Figure 20. DC Track Circuit Source: Adapted from Minor Railways Section Guideline on Train Detection DC Track Circuits, Figure 1, Institute of Railway Signal Engineers, 2014. Standard highway traffic signals always display a green, yellow, or red light except when power has failed and the signals are dark. Active crossing warning devices, however, usually incorporate some “fail-safe” design principles. In other words, the crossing warning system is designed to give an indication of an approaching train whenever the system fails. Crossing signals are normally dark unless a train is approaching or occupying the crossing, so there is no indication to the road user when power has failed. Therefore, crossing control systems include stand-by battery power should commercial power be terminated for any reason. Solar energy may be used to charge storage batteries to power signals at crossings in remote locations. Factors that may be considered in the design and installation of a train detection system include the following:
•Existing rail and ballast conditions
•V olume, speed, and type of highway and rail traffic
•Other train detection circuits that may be used on the same pair of rails for the regulation of train movements
•Train propulsion currents on electrified lines
•Track switch locations within the approach warning distances for a crossing
•Train detection circuits used for other crossings within the approaches (overlapping)
•Number of tracks
•Warning time and system credibility 53Constant Warning Time Track Circuit Where constant warning time is provided, the track circuitry equipment has the capability to sense a train in the approach section, measure its speed and distance from the crossing, and activate the warning equipment to provide the selected minimum warning time. Thus, regardless of train speed, a uniform warning time is provided. If a train stops prior to reaching the crossing or is moving away from the crossing, the warning devices are deactivated to allow highway traffic to move over the crossing. With constant warning time equipment, trains beyond 700 feet can move or switch on the approaches without reaching the crossing, and depending on their speed, not cause the crossing warning devices to be activated, thus eliminating unnecessary delays to highway traffic.
Warning Time Considerations
Train detection systems are designed to provide the minimum warning time for a crossing. The MUTCD and FRA regulations (49 CFR 234.225) require that the system provide for a minimum of 20 seconds of warning time. When determining if the minimum 20 seconds of warning time should be increased, some factors that should be considered include but are not limited to the following:
•Track clearance distances due to multiple tracks and/or angled crossings (add 1 second for each 10 feet of added crossing length in excess of 35 feet).
•The crossing is located within proximity of a highway intersection controlled by STOP signs where vehicles have a tendency of stopping on the crossing.
•The crossing is regularly used by long tractor-trailer vehicles.
•The crossing is regularly used by vehicles required to make mandatory stops before proceeding over the crossing (such as school buses and hazardous materials vehicles).
•The crossing’s active traffic control devices are interconnected with other highway traffic signal systems.
•It is necessary to provide at least 5 seconds between the time the approach lane gates to the crossing are fully lowered and when the train reaches the crossing (49 CFR 234.223).
•The crossing is regularly used by pedestrians and non-motorized components.
•The crossing and approaches are not level.
•Additional warning time is needed to accommodate a four-quadrant gate system.
•Other factors regarding crossing location as appropriate. It should be noted that even when constant warning devices are used, the calculated arrival time of the train at the crossing is based on the instantaneous speed of the train as it enters the crossing circuit. Once the calculation is made, changes in the train speed will change the train arrival time at the crossing and, correspondingly, reduce (or increase) the elapsed warning time at the crossing. This factor must be considered at a crossing interconnected to a nearby highway traffic signal utilizing either a simultaneous or advance preemption sequence. 54Excessive warning time has been determined to be a contributing factor in some collisions. Motorists who are stopped at an activated flashing ‑light signal and see no train approaching or see a distant train moving very slowly might ignore the warning of the signals and cross the tracks. A collision could result as the signals may have been activated by a high-speed passenger train just out of sight, not by the slower freight. If motorists are successful in clearing the tracks, they may assume that other crossings have excessive warning time. When they encounter a crossing with minimum warning time, they may ignore the signals, move onto the crossing, and become involved in a collision. This credibility problem is strengthened if motorists continue to successfully pass through activated signals with excessive warning time. Reasonable and consistent warning times reinforce system credibility. Unreasonable or inconsistent warning times may encourage undesirable driver behavior. Research has shown that when warning times exceed 40–50 seconds, drivers will accept shorter clearance times at flashing ‑lights and a significant number will attempt to drive around gates.(28) Although mandated maximum warning times do not yet exist, efforts should be made to ensure that traffic interruptions are reasonable and consistent without compromising the intended safety function of an active control device system’s design. Excessive warning times are associated with a permanent reduction in the class of track and/or train speeds without a concomitant change in the track circuitry or without constant warning time equipment. When not using constant warning train detection systems, track approach circuits should be adjusted accordingly when train speeds are permanently reduced. Another frequent cause of excessive warning times at crossings without constant warning time equipment is variable-speed trains, such as intercity passenger trains or fast commuter trains interspersed with slower freight trains. A major factor affecting system credibility is an unusual number of false activations at active crossings. Every effort should be made to minimize false activations through improvements in track circuitry, train detection equipment, and maintenance practices. A timely response to a system malfunction coupled with repairs made without undue delay can reduce credibility issues. Health monitoring systems capable of reporting to dispatch can improve response times. Design information about railroad interconnection circuits and approach length calculations can be found in the AREMA Communications and Signals Manual , Part 3.1.10, “Recommended Functional/Operating Guidelines for Interconnection Between Highway T raffic Signals and Highway-Rail Grade Crossing Warning Systems,” and Part 3.3.10, “Recommended Instructions for Determining Warning Time and Calculating Minimum Approach Distance for Highway-Rail Grade Crossing Warning Systems.” (29)