Direct Tube TrainSignal - EMC Storage Essentials
Myong Killings
Active traffic control devices are those that give advance notice of the approach of a train. 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. 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. Active traffic control devices include flashing light signals (both mast-mounted and cantilevered), bells, automatic gates, active advance warning devices, and highway traffic signals. Also included in this section is a description of the various methods of train detection.
Driving tasks at crossings with active traffic control devices differ somewhat from those at crossings with passive devices. Passive devices indicate that a crossing is present and that a highway user must 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. The motorist must take appropriate action when the devices are activated.
Active traffic control devices have proven an effective method of improving safety and operations at highway-railroad grade crossings. Effectiveness is the percentage reduction in collisions due to a crossing improvement. Utilizing data contained in the U.S. DOT National Highway-Rail Crossing Inventory and the Railroad Accident/Incident Reporting System databases, effectiveness factors have been developed for active devices. The effectiveness factors are shown in Table 40 along with results obtained from a California study and a study by William J. Hedley covering 23 years of experience on the Wabash Railroad.
The effectiveness factors presented in Table 40 were developed from before-and-after collision crash experience of groups of crossings actually improved. The same effectiveness would not necessarily be experienced at any other crossing where the same improvements (changes) were made. 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. Similar effectiveness could be anticipated under similar conditions.
Flashing light signals consist of two light units that flash alternately at a rate of 45 to 65 times per minute. Thus, like their predecessor, the wigwag, they simulate a watchman swinging a red lantern. Wigwags consist of a single red light unit that sways back and forth.
The main components of a flashing light unit are the hood, background, roundel, lamp, lampholder, reflector, and housing. The background is 20 or 24 inches in diameter and is painted a nonreflecting black to provide a contrast for the red light. The hood is also painted black.
The lamp consists of a low-wattage bulb used to ensure operation on stand-by battery power should commercial power fail. The wattage most commonly used is 18 or 25 watts; however, some railroads use quartz iodide bulbs of 16 or 36 watts. The reflector, or mirror, is mounted behind the lamp and directs the light back through the roundel.
Proper alignment of the light is essential. The lamp must be precisely aligned to direct the narrow intense beam toward the approaching motorist. The flashing light unit on the right-hand side of the highway is usually aligned to cover a distance far from the crossing. 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. Figures 22 and 23 show typical alignment patterns for a two-lane, two-way highway and for a multilane highway.
National warrants for the installation of flashing light signals have not been developed. Some states have established criteria based on exposure factors or priority indices. Other considerations include the following:
Post-mounted flashing light signals are normally located on the right side of the highway on all highway approaches to the crossing. Horizontal clearances for flashing light signals are discussed in the next section along with clearances for automatic gates.
Flashing light signals are generally post-mounted, but where improved visibility to approaching traffic is required, cantilevered flashing light signals are used. Cantilevered flashing lights may be appropriate when any of the following conditions exist:
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. However, two or more pairs of cantilevered flashing lights may be desirable for multilane approaches, as determined by an engineering study. The cantilevered lights can be placed over each lane so that the lights are mutually visible from adjacent driving lanes.
Cantilevers are available with fixed, rotatable, or walkout supports. 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. Rotatable cantilevers can be turned to the side of the highway for maintenance but not for aligning the flashing lights.
Most current installations utilize walkout cantilevers. The inclusion of a ladder and access walkway allows for easier maintenance with less impact to highway traffic. Standard cantilevers for mounting flashing lights are made with arm lengths up to 40 feet. Where cantilever arm length in excess of 35 feet is required, a bridge structure is preferred.
Additional pairs of light units may also be installed for side roads intersecting the approach highway near the crossing or for horizontal curves. Figure 27 shows the use of multiple pairs of lights to cover a horizontal curve to the left on the approach highway. 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.
Designers of LED systems should be aware of the voltage-current characteristics of the LED device they intend to use. The current versus voltage characteristic of an incandescent lamp is relatively linear over the normal operating range. At 5 volts, a 10-volt, 25-watt incandescent lamp draws approximately 1.8 amps. At 12 volts, it may draw 2.8 amps. Users should be aware that the current consumption of LED signals is dependent on the design of the LED array. Some LED flashing light units are resistive and, at 12 volts, may draw three times the current drawn at 10 volts. Other brands of LED flashing light units have power supplies designed to compensate for lower voltages. These lamps may draw more current when the voltage is less than 10 volts, which is a realistic concern during power outages. 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.
An automatic gate serves as a barrier across the highway when a train is approaching or occupying the crossing. 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. The light nearest to the tip burns steadily; the other two flash alternately. 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. 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.
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. Industry standards require that the gate arm shall start its downward motion not less than 3 seconds after the signal lights start to operate; shall reach its horizontal position before the arrival of the train; and shall remain in that position as long as the train occupies the crossing. 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. In the design of individual installations, consideration should be given to timing the operation of the gate arm to accommodate slow-moving trucks.
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. On multilane divided highways, an opening of approximately 6 feet may be provided for emergency vehicles.
Gates may be made of aluminum, fiberglass, or wood. Fiberglass or aluminum gates may be designed with a breakaway feature so that the gate is disengaged from the mechanism when struck. The American Railway Engineering and Maintenance-of-Way Association (AREMA) Communications and Signal Manual sets a limit of 38 feet for the gate length. 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. In these cases, crash cushions or other safety barriers may be desirable. Under no circumstances should signals or gate assemblies be placed in an unprotected painted median. Conversely, some railroads would prefer longer gate arms rather than a gate mechanism in the median. A typical clearance plan for a flashing light signal with automatic gate is shown in Figure 24.
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. 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. Where there is no curb or shoulder, the minimum horizontal clearance from the traveled way is 6 feet. Where flashing lights or gates are located in the median, additional width may be required to provide the minimum clearances for the counterweight support.
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