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16.08.2015
Active traffic control devices are those that give advance notice of the approach of a train. Driving tasks at crossings with active traffic control devices differ somewhat from those at crossings with passive devices. Active traffic control devices have proven an effective method of improving safety and operations at highway-railroad grade crossings. The effectiveness factors presented in Table 40 were developed from before-and-after collision crash experience of groups of crossings actually improved.
Flashing light signals consist of two light units that flash alternately at a rate of 45 to 65 times per minute.
The main components of a flashing light unit are the hood, background, roundel, lamp, lampholder, reflector, and housing. The roundel is red and comes in a variety of designs that direct the light toward the motorist. The lamp consists of a low-wattage bulb used to ensure operation on stand-by battery power should commercial power fail.
MUTCD provides that when indicating the approach or presence of a train, the flashing light signal shall display toward approaching highway traffic two red lights mounted in a horizontal line and flashing alternately. Post-mounted flashing light signals are normally located on the right side of the highway on all highway approaches to the crossing.
Flashing light signals are generally post-mounted, but where improved visibility to approaching traffic is required, cantilevered flashing light signals are used. A typical installation consists of one pair of cantilevered lights on each highway approach, supplemented with a pair of lights mounted on the supporting mast. Additional pairs of light units may also be installed for side roads intersecting the approach highway near the crossing or for horizontal curves.
Designers of LED systems should be aware of the voltage-current characteristics of the LED device they intend to use. An automatic gate serves as a barrier across the highway when a train is approaching or occupying the crossing. In a normal sequence of operation, the flashing light signals and the lights on the gate arm in its normal upright position are activated immediately upon the detection or approach of a train. In addition to the above factors, some states utilize a specified level of exposure or the priority index as a guideline for the selection of automatic gates.
The most recent criteria for the use of automatic gates are provided in the guidance document prepared by the U.S. On two-way streets, the gates should cover enough of the approach highway to physically block the motorist from driving around the gate without going into the opposing traffic lane. When no train is approaching or occupying the crossing, the gate arm is held in a vertical position and the minimum clearance from the face of the vertical curb to the nearest part of the gate arm or signal is 2 feet, for a distance of 17 feet above the highway.
The lateral location of flashing light and gate assemblies must also provide adequate clearances from the track as well as space for construction of the foundations. Figures 30 through 36 show typical location plans for flashing light signals with and without gates. Four-quadrant gate systems consist of a series of automatic flashing light signals and gates in which the gates extend across both the approach and the departure side of roadway lanes. At this time, only a small number of four-quadrant gate systems have been installed in the United States, and they incorporate different types of designs to prevent vehicles from being trapped between the gates. Four-quadrant gates are recognized as a supplemental safety measure under the Final Rule for quiet zones (refer to Chapter II, Section B.) It should be noted that FRA has assigned a lower effectiveness to installations that include vehicle presence detection because the act of raising the exit gates may allow vehicles to enter the crossing. Despite the dangers of crossing in front of oncoming trains, drivers continue to risk lives and property by driving around crossing gates. Limitations are common to the use of any form of traffic separation at highway-rail grade crossings. It should be noted that median treatments meeting the requirements of 49 CFR 222 are considered supplemental safety measures by FRA for use in a quiet zone (refer to Chapter II, Components of a Highway-Rail Grade Crossing).
Various styles of median treatments include barrier wall systems, wide raised medians, and mountable raised curb systems.
The state of Illinois has developed a standard that uses a combination of mountable and non-mountable curbs to provide a wide raised median with escape zones both in the median as well as to the shoulder (see Figure 37). Because mountable curbs are made to allow emergency vehicles to cross and designed to deflect errant vehicles, they also are the easiest of all the barriers and separators to violate. These devices have proven a low-cost investment with a high rate of return in safety at crossings. The use of median separators at the Sugar Creek Road crossing in the North Carolina Sealed Corridor Program has resulted in a 77-percent reduction in crossing violations.
The barrier gate is a movable automatic gate designed to close an approaching roadway temporarily at a highway-rail crossing.
Barrier gates could be applied to situations requiring a positive barrier, such as in a down position, closing off-road traffic, and opening only on demand. A crossing bell is an audible warning device used to supplement other active traffic control devices. The wayside horn system consists of a horn or series of horns located at the highway-rail grade crossing and directed at oncoming motorists. The purpose of the wayside horn system is to focus the horn sound level on the road user while minimizing the noise impact adjacent to the railroad from the point the train horn is required to be sounded. The system is used as an adjunct to train-activated warning systems to provide audible warning of an approaching train for traffic on all approaches to the highway-rail grade crossing. When a wayside horn system is used at highway-rail grade crossings where the locomotive-mounted horn is not sounded, the highway-rail grade crossings must be equipped with flashing lights and gates and constant warning circuitry, where practical. The wayside horn system simulates a train horn and sounds at a minimum of 15 seconds prior to the train's arrival at the highway-rail grade crossing, or simultaneously with the activation of the flashing lights or descent of the gate, until the lead locomotive has traversed the crossing.
At its June 2006 meeting, the NCUTCD council approved a proposed new section to Part 8 of MUTCD to recognize use of the wayside horn either as a supplemental audible device or as an alternative to the sounding of a locomotive-mounted horn. The active advance warning sign (AAWS) consists of one or two 12-inch yellow hazard identification beacons mounted above the advance warning sign, as shown in Figure 39.
A train-activated advance warning sign should be considered at locations where the crossing flashing light signals cannot be seen until an approaching motorist has passed the decision point (the distance from the track from which a safe stop can be made). To allow the traffic queue at the crossing time to dissipate safely, the advance flashers should continue to operate for a period of time after the active control devices at the crossing deactivate, as determined by an engineering study. If such an advance device fails, the driver would not be alerted to the activated crossing controls.
AAWS should be placed at the location where the advance warning sign would normally be placed. At a signalized intersection located within 60 meters (200 feet) of a highway-rail grade crossing, measured from the edge of the track to the edge of the roadway, where the intersection traffic control signals are preempted by the approach of a train, all existing turning movements toward the highway-rail grade crossing should be prohibited during the signal preemption sequences.
During the time this handbook was being updated, NCUTCD and FHWA were considering a proposal to amend MUTCD to include a new traffic signal warrant that would apply, under certain conditions, to highway-highway intersections in close proximity to highway-railroad grade crossings. When applied, any traffic signals installed pursuant to this new warrant would also need to include provisions for railroad preemption (for example, if not already existing, some means of automatically detecting a train approaching the highway-railroad grade crossing would also need to be provided), to allow for clearing any queued vehicles off the grade crossing prior to the arrival of a train. Draft language proposing the new warrant was approved by the National Council at the June 2006 meeting of NCUTCD. To serve their purpose of advising motorists and pedestrians of the approach or presence of trains, active traffic control devices are activated by some form of train detection.
Standard highway traffic signals display a green, yellow, or red light at all times except when power has failed and the signals are dark.
On tracks where trains operate at speeds of 20 mph or higher, the circuits controlling automatic flashing light signals shall provide for a minimum operation of 20 seconds before the arrival of any train.
The traffic engineer designing the preemption system must understand how the traffic signal controller unit operates in response to a call for a preemption sequence.
Preemption of traffic signals for railroad operations is very complex and must be designed and operated for a specific location, often with unique conditions.
A crossing equipped with a passive control device may need to be upgraded to include active warning devices so that preemption of the traffic signal can be implemented effectively. Where a crossing with active control devices is in close proximity to a STOP-sign controlled intersection, it may be necessary to consider the installation of traffic signals to clear queues from the crossing if an engineering study indicates that other solutions or traffic control devices will not be effective.
Where possible, field observations of traffic queue lengths during critical traffic periods can provide guidance on the need for signal preemption.
The factor of 25 represents the effective length of a passenger car (vehicle length plus space between vehicles); the factor of 2 is a random arrival factor.
It is not the intent of either MUTCD or the queue calculation equation to provide a specific distance as the sole criterion to interconnect railroad and highway signals. A long, slow-moving truck turning toward the tracks could have a problem clearing the intersection if a simultaneous preemption call occurs at the beginning of its turn, especially where the distance between the intersection and the vehicle stop line for the crossing is very short. A long truck or a vehicle required to stop before crossing the tracks in low gear could have a problem clearing a lowering gate as well as clearing the intersection. Special studies may be needed to determine if traffic approaching the crossing could queue and eventually block the adjacent intersection traffic flow. Equations 1 and 2 c an a lso be used to estimate the queue length that is likely to develop for traffic approaching the railroad crossing.
The factor of 35 assumes that approximately 25 seconds before the train enters the crossing plus 10 seconds after it clears the crossing, the crossing would still be blocked by the gates. The preemption interconnect may consist of simultaneous preemption (traffic signals are preempted simultaneously with the activation of the railroad control devices), advance preemption (traffic signals are preempted prior to the activation of the railroad control devices), or, possibly, a special design that could consist of two separate closed-loop normally energized circuits. One preemption interconnect circuit can be used to initially clear out the pedestrian traffic, then a time delay is used for the second vehicular clearance. If the pedestrian clearance phase is abbreviated (or eliminated), additional signing alerting pedestrians of a shortened pedestrian cycle should be considered.
Until recently, most traffic signal controllers were unable to recognize a second preempt until the entire preemption sequence of the first activation timed out. The point at which preemption is released from the railroad active control devices to the traffic signals is critical to the proper operation of re-service.
Some traffic signal controller manufacturers have designed systems in which the critical railroad preemption parameters can not be changed without both proper software and physically making a hardwire change in the traffic signal cabinet. One relay, the preemption relay, is energized only when the railroad active control devices are off.
In all cases, remote monitoring devices that send alarm messages to the railroad and highway authority should be installed.
You can get creative with these switches and have them activate relay switches at the same time that they activate your turnout so that lights come on or animations occur. They are 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.
Passive devices indicate that a crossing is present and that a highway user must look for an approaching train and take appropriate action.
The warning system is designed to give an indication of an approaching train whenever the system has failed. The same effectiveness would not necessarily be experienced at any other crossing where the same improvements (changes) were made.
DOT Technical Working Group guidance document provides guidelines for selecting active devices (see Chapter V).
The background is 20 or 24 inches in diameter and is painted a nonreflecting black to provide a contrast for the red light.
The wattage most commonly used is 18 or 25 watts; however, some railroads use quartz iodide bulbs of 16 or 36 watts. The lamp must be precisely aligned to direct the narrow intense beam toward the approaching motorist.
Flashing light signals shall be placed to the right of approaching highway traffic on all highway approaches to a highway-rail grade crossing. Horizontal clearances for flashing light signals are discussed in the next section along with clearances for automatic gates. However, two or more pairs of cantilevered flashing lights may be desirable for multilane approaches, as determined by an engineering study. The primary disadvantage of the fixed support is that maintenance of the light unit is usually performed from equipment in the traffic lane, thereby blocking highway traffic. The inclusion of a ladder and access walkway allows for easier maintenance with less impact to highway traffic. Figure 27 shows the use of multiple pairs of lights to cover a horizontal curve to the left on the approach highway. The current versus voltage characteristic of an incandescent lamp is relatively linear over the normal operating range. The gate is reflectorized with 16-inch diagonal red and white stripes.90 To enhance visibility during darkness, three red lights are placed on the gate arm.


On multilane divided highways, an opening of approximately 6 feet may be provided for emergency vehicles. Fiberglass or aluminum gates may be designed with a breakaway feature so that the gate is disengaged from the mechanism when struck.
Where there is no curb, a minimum horizontal clearance of 2 feet from the edge of a paved or surfaced shoulder is required, with a minimum clearance of 6 feet from the edge of the traveled highway. Figure 29 shows typical locational requirements for the foundations for flashing lights and cantilevered flashing lights with gates. If it is necessary to locate the supporting post in a potentially hazardous position to ensure adequate visibility, some type of safety barrier should be considered.
Unlike two-quadrant gate systems, four-quadrant gates provide additional visual constraints and inhibit nearly all traffic movements over the crossing after the gates have been lowered. In some installations, the exit gates are delayed to allow roadway vehicles to clear before the crossing is secured; other systems include vehicle presence detection to hold the exit gates up while vehicles are within the crossing zone. At many crossings, drivers are able to cross the centerline pavement marking and drive around a gate with little difficulty.
These include restricting access to intersecting streets, alleys, and driveways within the limits of the median, and possible adverse safety effects. Concrete barrier walls and guardrails generally prevent drivers from crossing into opposing lanes throughout the length of the installation.
Additions such as trees, flowers, and other vegetation higher than .9 meter (3 feet) above the roadway can restrict drivers' view of approaching trains.
Non-mountable curb islands are typically 6 to 9 inches in height and at least .6 meter (2 feet) wide and may have reboundable, reflectorized vertical markers. Mountable raised curb systems with reboundable vertical markers present drivers with a visual impediment to crossing to the opposing traffic lane.
The separators are installed along the centerline of roadways, in most cases extending approximately 20 to 30 meters (70 to 100 feet) from the crossing. The use of median separators in conjunction with four-quadrant gates has produced a 98-percent reduction in crossing violations. A typical installation includes a housing containing electromechanical components that lower and raise the gate arm, the arm itself, and a locking assembly bolted to a concrete foundation to receive and hold the lowered gate arm in place.
The system is designed on fail-safe principles and provides a means to verify sound output. It is not required to direct the wayside horn system toward approaching roadway users from roadways adjacent to the railroad if the roadway users' movements toward the crossing are controlled by a STOP sign or traffic signal. Where multiple tracks are present, the wayside horn system is immediately reactivated when another train is detected before the previous train clears the crossing. The council also approved new language for Part 10, which allows use of the wayside horn for light rail. An advisory speed plate sign indicating the safe approach speed also should be posted with the sign.94 The AAWS provides motorists with advance warning that a train is approaching the crossing. To enhance visibility at crossings with unusual geometry or site conditions, the devices may be cantilevered or installed on both sides of the highway. This sign is used on a limited basis, normally near commuter stations where multiple tracks and high volumes of pedestrian traffic are present. This proposed warrant or some version thereof is likely to be included in a formal Notice of Proposed Amendment to MUTCD, which is currently expected to be issued by FHWA in late 2007 or early 2008. A preemption sequence compatible with railroad crossing active traffic control devices is extremely important to provide safe vehicular and pedestrian movements. Generally, the method is automatic and requires no personnel to operate it, although a small number of such installations are operated under manual control.
Crossing signals are normally dark unless a train is approaching or occupying the crossing. The stand-by assures normal operation of crossing signals during a commercial power outage. This ability of modern rectifiers permits DC operation of the signals whether AC supply voltage is present or not. The engineer must consult with railroad personnel who are responsible for railroad signal design and operations to ensure that appropriate equipment is specified and that both highway and railroad signal installations operate properly and with full compatibility.
With the extremely large number of variables involved, it is difficult to simply quantify all the time and distance elements. Queue arrival and dissipation studies should be made during peak travel demand times at the site.
Special consideration should be given where upstream signals cause vehicles to arrive in platoons that could result in long queue lengths. Where the clear storage distance between the crossing and the highway intersection stop line is not sufficient to safely store a design vehicle (typically the longest legal truck combination), or if vehicles regularly queue across the tracks, a pre-signal should be considered. The placement of train detection equipment should be governed by the preemption time required to clear the queues. If the truck makes the turn, encounters a lowering gate, and stops in compliance with the gate, the exit path from the crossing for vehicles approaching the intersection may be blocked even though the traffic signal preemption is functioning and displaying track clearance green. Both of these scenarios should be considered in the design of the preemption operation if there is a significant volume of trucks. The factor q then represents the flow rate (per lane) approaching the crossing including both traffic passing straight through the signalized intersection toward the crossing as well as traffic that turns left and right off the street that parallels the tracks. MUTCD requires a 20-second minimum time for the railroad circuit to activate warning devices prior to arrival of a through train.
Because some of these vehicles are required to stop and proceed in low gear across the tracks, clearance time for both the tracks and the signalized intersection must be considered.
This is accomplished by requiring additional clearance time from the railroad for simultaneous preemption.
MUTCD points out the need for a systems approach when designing, installing, and operating highway traffic signals interconnected to railroad crossings.
They also interact with lesser priority preemption programs from emergency and other special highway vehicles. CWT is particularly useful where trains travel at significantly different speeds or frequently stop within the control circuit limits (useful in commuter and switching operations). The supervised circuitry can detect if the interconnect circuit is open or wires are crossed and set the traffic signals to flashing operation or send in an alarm. Standardizing the interface will allow interoperability between wide varieties of equipment. The first, a pedestrian clearance call, should occur at a predetermined length of time to be defined by a traffic engineering study and should continue until the train has departed the crossing.
A system with two separate circuits provides a more uniform timing if the train speed varies once preemption occurred. If the second demand occurred during the initial preemption sequence, the traffic signal controllers continued the same sequence as if that was still the initial demand for preemption.
For the traffic signal controller to recognize a second demand, the first demand must be released. Security of programmed parameters is critical to the proper operation of the highway-rail preemption system. Without proper data changes, the traffic signals will remain in a flashing red operation until the data are corrected.
The interconnection circuit between the highway traffic signal control cabinet and the railroad signal cabinet should be designed as a system.
In its simplest form, the supervised circuit has two control relays in the traffic control cabinet, each of which is energized by the railroad crossing relay (see Exhibit 3). The second relay, the supervision relay, is energized only when the railroad active control devices are operating.
Active traffic control devices are supplemented with the same signs and pavement markings used for passive control, except that STOP or YIELD signs shall not be used where active traffic control devices are installed.
At crossings with active traffic control devices, a motorist is told when a train is approaching.
It should be remembered that, in those studies, the crossings were selected for improvement by competent authorities as a precondition to performance of the work. A deflecting roundel directs a portion of the light from the beam to one side of the axis in the direction indicated on the lens.
The reflector, or mirror, is mounted behind the lamp and directs the light back through the roundel.
The flashing light unit on the right-hand side of the highway is usually aligned to cover a distance far from the crossing.
They shall be located laterally with respect to the highway in conformance with Figure 24, except where such location would adversely affect signal visibility.
The cantilevered lights can be placed over each lane so that the lights are mutually visible from adjacent driving lanes.
Rotatable cantilevers can be turned to the side of the highway for maintenance but not for aligning the flashing lights. A horizontal curve to the right may be covered by placing another roadside flashing light unit on the opposite side of the highway, as shown in Figure 28. When the train clears the crossing, and no other train is approaching, the gate arm shall ascend to its upright position normally in not more than 12 seconds, following which the flashing lights and the lights on the gate arm shall cease operation.
The American Railway Engineering and Maintenance-of-Way Association (AREMA) Communications and Signal Manual sets a limit of 38 feet for the gate length. Where there is no curb or shoulder, the minimum horizontal clearance from the traveled way is 6 feet. The area for the foundation and excavation must be analyzed to determine the effect on sidewalks, utility facilities, and drainage.
The number of crossing gate violations can be reduced by restricting driver access to the opposing lanes. The median should be designed to allow vehicles to make left turns or U-turns through the median where appropriate, based on engineering judgment and evaluation.
Although they do not present a true barrier, wide medians can be nearly as effective because a driver would have significant difficulty attempting to drive across to the opposing lanes. Drivers have significant difficulty attempting to violate these types of islands because the 6- to 9-inch heights cannot be easily mounted and crossed.
The curbs are no more than 6 inches in height, less than 12 inches in width, and built with a rounded design to create minimal deflection upon impact. Care should be taken to assure that the system maintains its stability on the roadway with design traffic conditions and that retroreflective devices or glass beads on the top and sides of the curb are maintained for night visibility. The barrier gate arm itself has been installed with a system consisting of three steel cables, the top and bottom of which are enclosed aluminum tubes.
FRA has indicated that a barrier gate, if equipped with monitoring and confirmation as required by the Final Rule, may be applicable to enforce a nighttime closure for partial quiet zones. Bell circuitry may be designed so that the bell stops ringing when the lead end of the train reaches the crossing.
Wayside horn systems should include a 3- to 5-second delay after activation of flashing lights signals before sounding. The text in Part 10 would allow a wayside horn to be used to reproduce the tone and sound level of wayside equipment. The beacons are connected to the railroad track circuitry and activated on the approach of a train.
Consideration should be given to providing a back-up source of power in the event of commercial power failure. The location of this supplemental advance warning sign is dependent on vehicle speed and the geometric conditions of the roadway. The sign will activate when a train is located within the crossing's approach circuits and a second train approaches the crossing. Such preemption serves to ensure that the actions of these separate traffic control devices complement rather than conflict with each other. When this practice was initiated, the crossing signals were normally supplied with AC power through a step-down transformer.
The signals are connected directly to battery terminals and the power transfer is eliminated, as shown in Figure 40.
The warning time should be of sufficient length to ensure clearance of a vehicle that might have stopped at the crossing and then proceeded to cross just before the flashing lights began operation. Continuous cooperation between highway and railroad personnel is essential for safe operation.
The goal of this recommended practice is to identify as many elements as possible and provide references where feasible. Where field observation is not possible because the crossing is not yet in full operation, some intersection capacity analysis computer programs that provide an estimate of queue lengths can be used to determine whether the 95th-percentile queue from the signalized intersection will extend as far as the railroad crossing.


This time should take into account the critical or design vehicle and should be sufficient for this vehicle to clear the intersection safely before the arrival of the train. This condition should be studied as part of the system design and, if warranted, advance preemption should be employed to allow adequate time for a truck to clear prior to activating the railroad warning devices. In this case, additional gate delay time may be necessary to allow these vehicles adequate time to restart and clear the crossing prior to lowering of the gates. Note, however, that excessive clearance time may result in increased violation of lowered gates by motorists.
The Traffic Control Devices Handbook describes a diagnostic team that may include persons representing highway, railroad, regulatory, and utility agencies as well as manufacturers of highway and railroad equipment. This is especially important if the train accelerates after the pedestrian clearance is initiated. In other words, if a traffic signal controller receives an initial preempt activation and shortly thereafter is deactivated, most traffic signal controllers will continue to time out the preemption sequence; if a second demand for preemption is placed during this period, the traffic signal controller must return to the track clearance green.
The traffic signal controller re-service capability must be able to accept and respond to any number of demands for preemption. Therefore, the railroad active control devices must release the preempt activation just as the crossing gates begin to rise, not when they reach a fully vertical position. As an absolute minimum, control equipment cabinets should be locked and secure to prevent tampering, and controllers should be password protected. In addition, these systems prevent a different type of controller or controller software from operating the traffic signals. Frequently, the interconnect cable circuit is designed so that the preemption relay can be falsely de-energized, thereby causing a preempt call without the railroad signals being activated. That way you can line up your turnouts with the throw of one switch to allow your train to follow a particular route. Active traffic control devices include flashing light signals (both mast-mounted and cantilevered), bells, automatic gates, active advance warning devices, and highway traffic signals. A roundel having both spreadlight and deflecting features is designed so that the deflection is at a right angle to the spread. The light units mounted on the back of the signals on the opposing approach and, thus, on the left, are usually aligned to cover the near approach to the crossing.
At highway-rail grade crossings with highway traffic in both directions, back-to-back pairs of lights shall be placed on each side of the tracks. Where cantilever arm length in excess of 35 feet is required, a bridge structure is preferred. The gate is combined with a standard flashing light signal (see Figure 29 for a typical installation) that provides additional warning before the arm starts to descend, while the gate arm is across the highway, and until the gate arm ascends to clearance. In the design of individual installations, consideration should be given to timing the operation of the gate arm to accommodate slow-moving trucks. Some railroads request reconfiguration of the crossing when gate arm lengths would exceed 32 feet and it may be necessary to place gate assemblies in the median to cover the approach highway. Where flashing lights or gates are located in the median, additional width may be required to provide the minimum clearances for the counterweight support. Although these plans indicate a 12-foot minimum clearance between the center of the flashing light assembly and the center of the tracks, some railroads prefer a 15-foot minimum clearance. Clearly, large multilane intersections and intersections with unusual configurations will require careful study to determine the appropriate layout of crossing gate locations. Highway authorities have implemented various median separation devices, which have shown a significant reduction in the number of vehicle violations at crossing gates. However, the road must be wide enough to accept the width of the barrier and the appropriate end treatment. When used together, the mountable raised median and vertical delineators discourage passage. When gates are used, the bell may be silenced when the gate arms descend to within 10 degrees of the horizontal position. This would allow use of directional horns in lieu of traditional crossing bells at locations with light rail not subject to FRA jurisdiction, such as urban light-rail crossings. It is also being evaluated at multiple-track highway-rail grade crossings as a supplement to automatic gates. This electrical circuit uses the rails as conductors in such a way that the presence of a solid electrical path, as provided by the wheels and axles of a locomotive or railroad car, shunts the circuit. Therefore, crossing control systems are designed to also operate on stand-by battery power should commercial power be terminated for any reason. The same AC source provided charging current through a rectifier for the stand-by battery to maintain the battery in a charged condition. Important information concerning the type of railroad signal equipment that can be used is available from the operating railroad and from the AREMA Communications and Signal Manual. Recommendations are therefore provided in the generic sense, with the expectation that applications will be designed for local conditions. Vehicle classification studies should be performed, because trucks must be factored separately, and some trucks may have unusual size and operating characteristics.3,4 Similar locations may be evaluated for comparative vehicle queuing.
In addition, traffic signal phases conflicting with the crossing can be omitted from the preemption phasing sequence. The importance of cooperation and interaction among all responsible parties cannot be emphasized enough. The second, a vehicle clearance call, programmed with a higher priority in the traffic signal controller than the first call, should occur at a predetermined length of time to be determined in a traffic engineering study, but not less than 20 seconds prior to the arrival of a train, and should continue until the train departs the crossing. At any point in the preemption sequence, even during the track clear green interval, the controller must return to the start of a full track clearance green interval with a second preemption demand.
Otherwise, especially at locations with short storage areas between the crossing and the highway intersection, traffic may creep under the rising gates and, with a second train, a second track clear green interval will not be provided if the gates never reach a fully vertical position. In addition to preventing malicious tampering with control devices, security should be considered to prevent accidental changes in timing parameters, especially in the traffic signal controller, where a programming mistake can easily be made due to the large quantity of parameters, even when just viewing the data. It is important to preserve the integrity of the system once it is tested and proven to operate properly. If both relays are simultaneously energized or de-energized, the supervision logic determines that there is a problem and can implement action.
The effectiveness factors are shown in Table 40 along with results obtained from a California study and a study by William J.
Figures 22 and 23 show typical alignment patterns for a two-lane, two-way highway and for a multilane highway.
On multilane one-way streets and divided highways, flashing light signals shall be placed on the approach side of the highway-rail grade crossing on both sides of the roadway or shall be placed above the highway. Users should be aware that the current consumption of LED signals is dependent on the design of the LED array. The gate mechanism is either supported on the same post with the flashing light signal or separately mounted on a pedestal adjacent to the flashing light signal post. For such conditions, gate arm requirements may become a principal factor in the layout of the intersection geometry and channelization from the outset.
Sight restrictions for vehicles with low driver eye heights and any special needs for emergency vehicles to make a U-turn maneuver should be considered (but not for the purpose of circumventing the traffic control devices at the crossing). A wide median, if attractively landscaped, is often the most aesthetically pleasing separation method. These systems are designed to allow emergency vehicles to cross opposing lanes (but not for the purpose of circumventing the traffic control devices at the crossing). Attached to the islands are flat delineator panels or tubes with reflectorized taping for better visibility at night. Silencing the bell when the train reaches the crossing or when the gates are down may be desired to accommodate residents of suburban areas. Solar energy may be used to charge storage batteries to power signals at crossings in remote locations.
When commercial AC power failed, crossing signal power connections were transferred from the AC source to the battery, as shown in Figure 45.
Factors that can affect this time include the width of the crossing, the length and acceleration capabilities of vehicles using the crossing, highway grades, and the condition of the crossing surface. The list of conditions requiring preemption is not intended to be complete but should provide an awareness of the factors necessitating preemption of normal traffic signal operation. The current edition of MUTCD also mentions that coordination with the flashing light system should be considered for traffic signals located farther than 200 feet (60 meters) from the crossing.1 Coordination could include, for example, queue detection that would omit some signal phases or activate variable message signs.
Such cooperation not only encourages the safest design available by combining the latest technology available (or under development) in highway and railroad equipment but also ensures proper operation. Drivers may yield to pedestrians and, thereby, prevent vehicles behind them from clearing off the tracks.
The purpose of the second call is to clear motor vehicle queues, which may extend into the limits of the crossing. Another method of preserving the proper timing parameters is remote monitoring of the traffic signal controller. If a train approaches the crossing during the false preemption, the railroad signals will activate, but the traffic signals will not provide track clearance phases because they are still receiving the first false call. A roundel using a 20-degree spread and 32-degree downward deflection can be used on cantilevers.
A crossbuck is always used in conjunction with the flashing light signal and is usually mounted on the same post above the light units.
Some LED flashing light units are resistive and, at 12 volts, may draw three times the current drawn at 10 volts.
Under no circumstances should signals or gate assemblies be placed in an unprotected painted median. The crossing gate (and, therefore, traffic control) treatment should be an integral part of the design of an intersection, not an afterthought. Installation lengths can be more effective if they extend beyond a minimum length of 46 meters (150 feet). Care should be taken to assure that an errant vehicle cannot bottom-out and protrude into the oncoming traffic lane. The delineator panels are flexible yet securely anchored to return to their original positions if struck by a vehicle.
See the following section for additional information regarding the application and design of pre-signals. To minimize this potential, full pedestrian clearance may be provided but, consequently, additional minimum preemption warning time will be required.
Routine uploads of traffic signal timings can be compared to a database to check for unapproved changes in any timing parameters. Even worse, a short between the wires in this type of circuit will virtually disable preemption and will only be recognizable once the railroad active control devices are activated with an approaching train. Upon completion of the clear-out, the traffic signals can go into an all-way flashing red instead of stop. Other brands of LED flashing light units have power supplies designed to compensate for lower voltages. Conversely, some railroads would prefer longer gate arms rather than a gate mechanism in the median. Sight restrictions for low driver eye heights should be considered if vertical markers are installed.
The all-way flashing red will allow traffic to advance off the tracks instead of being held by the red signal. These lamps may draw more current when the voltage is less than 10 volts, which is a realistic concern during power outages. A typical clearance plan for a flashing light signal with automatic gate is shown in Figure 24.
Access requirements should be fully evaluated, particularly allowing emergency vehicles to cross opposing lanes (but not for the purpose of circumventing the traffic control devices at the crossing).
An engineering study may determine that the all-way flashing red is undesirable due to high highway traffic volumes compared to rail traffic. Someone asked me a question recently about how to wire these momentary SPDT toggle switches to the Atlas remote switch machine. To avoid damaging control circuits, which may result in dark signals, designers of LED flashing light signal circuits should consider the maximum current drawn by LED units over the expected voltage range. If it's not a momentary switch, you will burn out the solenoid switch machine the first time you use it.




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Comments to «Model railroad wiring lights»

  1. SEMIMI_OQLAN writes:
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  3. EPISODE writes:
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