U.S. Department of Transportation
Federal Highway Administration
1200 New Jersey Avenue, SE
Washington, DC 20590
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In addition to the installation of traffic control systems, site and operational improvements can significantly contribute to the safety of highway-rail crossings. Site improvements are discussed in four categories: crossing geometry, removing obstructions, illumination, and safety barriers.
The ideal crossing geometry is a 90-degree intersection of track and highway with slight-ascending grades on both highway approaches to reduce the flow of surface water toward the crossing. Few crossings have this ideal geometry because of topography or limitations of ROW for both the highway and the railroad. Every effort should be made to construct new crossings in this manner. Horizontal and vertical alignment and cross-sectional design are discussed below.
Ideally, the highway should intersect the tracks with a minimum skew with no nearby intersections or driveways. This layout enhances the driver's view of the crossing and tracks and reduces conflicting vehicular movements from crossroads and driveways. To the practical extent, crossings should not be located on either highway or railroad curves. Roadway curvature inhibits a driver's view of a crossing ahead, and a driver's attention may be directed toward negotiating the curve rather than looking for a train. Railroad curvature inhibits a driver's view down the tracks from both the approach to the crossing and from a stopped position at the crossing. Crossings located on both highway and railroad curves present maintenance problems and poor rideability for highway traffic if conflicting superelevation is present. Similar difficulties arise when superelevation of the track is opposite to the grade of the highway.
If the intersection between track and highway cannot be made at right angles, the variation from 90 degrees should be minimized. At skewed crossings, motorists must look over their shoulders to view the tracks. Because of this more awkward movement, some motorists may only glance quickly and not take necessary precautions.
Improvements to horizontal alignment can be expensive, depending on the extent of construction required. Special consideration should be given to crossings that have complex horizontal geometries, as described previously. These crossings may warrant the installation of active traffic control systems or, if possible, may be closed to highway traffic.
Crossing Profile –Vertical Alignment
It is desirable that the intersection of highway and railroad be as level as possible from the standpoint of sight distance, rideability, and braking and acceleration distances. Positive drainage is provided if the crossing is located at the peak of a vertical curve on the highway; however, the curve should be adequately flat to avoid hanging-up of vehicles and of sufficient length to ensure an adequate view of the crossing consistent with the highway design or operating speed.
When constructing new highway-rail at-grade crossings or enhancing existing locations, care should be taken to create horizontal and vertical profiles that provide smooth and safe travel for motorists approaching and using crossings. Vehicles or trailers low to the ground relative to the distance between axles pose the greatest risk of becoming stuck at crossings due to contact with the track or highway surface. Similarly, a low vehicle's front or rear bumper overhang may strike or drag along the pavement surface in a sag vertical curve.
The AASHTO presents a guideline which is traversable by a wide range of vehicles including those with long wheelbases and/or low ground clearance (this standard is also provided by AREMA). Shown in Figure 1, the guideline recommends that the crossing surface be in the same plane as the top of rails for a distance of 2 feet outside of the rails, and that the surface of the highway be not more than 3 inches higher or lower than the top of the nearest rail at a point 30 feet from the rail for new construction.
Figure 1. Highway-Rail Crossing Cross Section
Source: Figure 9-102, A Policy on Geometric Design of Highway and Streets, Washington, DC, AASHTO, 2018. Used by permission.
Low-clearance vehicles pose the greatest risk of becoming immobilized at highway-rail crossings due to contact with the track or highway surface. Except for specialized vehicles such as tank trucks, there is little standardization within the vehicle manufacturing industry regarding minimum ground clearance. Instead, the requirements of shippers and operators guide manufacturers.(17)
Two constraints often apply to the maintenance of crossing profiles: drainage requirements and resource limitations. Coordination of maintenance activities between rail and highway authorities, especially at the city and county levels, can be frequently informal and unstructured. Even when the need to coordinate has been identified, there may be a lack of knowledge regarding whom to contact. For these reasons, it is important to note that with any routine track work, or any highway/roadway surface repaving work to be performed, notification of all parties, and acknowledgement of receipt of notice, should occur prior to any work commencing.
Existing crossings constructed on an embankment for drainage purposes may be problematic for low ground clearance vehicles to negotiate. Historically, track maintenance may have raised the track over time if additional ballast was placed beneath the ties. Unless the highway profile was also adjusted, this practice can result in a "humped" profile that may adversely affect the safety and operation of highway traffic over the railroad. Modern maintenance equipment and practices re-set the track structure and "tamp" the ballast in place without modifying the track elevation.
In some cases, highway authorities become aware of increases in track elevation only after maintenance activities have taken place. As a result, even if State standards exist, there is little opportunity to enforce them. Often, an individual increase in track elevation may not violate a guideline, but successive track raises may slowly create a high-profile crossing. Over time, this can result in a condition referred to as a "hump crossing."
Strategies to address this problem could include the following:
Because of the previously noted variations in vehicle configurations, it is difficult to determine whether a crossing which does not meet the AASHTO guideline is traversable by all long wheelbase and low ground clearance vehicles. Also, there are many crossings which do not meet the AASHTO guideline exactly but nevertheless are unlikely to strand low ground clearance vehicles.
The Florida Department of Transportation (FDOT) published Design Guidelines for Highway Railroad Grade Crossing Profiles in Florida.(18) This document provides an in-depth discussion of the technical issues, lists various standards in use, and presents more practicable solutions for development of crest and sag curves which would prevent hang-up of low clearance vehicles. Figure 2 shows a crest curve treatment using three vertical curves. The project also resulted in the development of software known as HANGUP for evaluation of crossing profiles.
At locations where the profile is sufficiently abrupt to potentially immobilize vehicles, a Low Ground Clearance Grade Crossing (W10-5) warning sign and a LOW GROUND CLEARANCE (W10-5P) supplemental plaque, Figure 3, may be installed for each direction of travel to warn drivers of long wheelbase vehicles or drivers of vehicles that have a low ground clearance that they might encounter a hang-up situation if they attempt to use the crossing. (Figure 4 shows a typical treatment.) The USDOT Crossing Inventory Form(19), in Part III, Box 2E, provides a place to indicate if a crossing has such signs installed.
Figure 2. Fitting Three Vertical Curves to an Approach to a Railroad Crossing Profile
Source: Sobanjo, J., Design Guidelines for Highway Railroad Grade Crossing Profiles in Florida, Figure 4.6, Florida State University, May 2006.
Figure 3. Low Ground Clearance Warning Signs
Source: Manual on Uniform Traffic Control Devices, 2009 Edition.
Figure 4. Treatment for Low Ground Clearance
Source: Seyfried, R K., (Ed.), Traffic Control Devices Handbook, 2nd Edition, Figure 11-5, Washington, DC, ITE, 2013.
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