Roadway Design Standards To Accommodate Low-Clearance Vehicles

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TRANSPORTATION RESEARCH RECORD 1356

Roadway Design Standards To Accommodate Low-Clearance Vehicles

w. s. RONALD

ECK AND K. KANG

It has been attempted to develop geometric design standards to accommodate low-ground-clearance vehicles using computer software. Low-clearance vehicles include lowboy equipment trailers, car carriers, single- and double-drop van trailers, and cars and trucks with trailers. Hang-ups and overhang dragging on highprofile roadways are causes of concern for low-ground-clearance vehicles. The objective was achieved through the development and application of the HANGUP software package and the analysis of the design standards of several agencies. Although a few agencies have developed geometric design standards for lowclearance vehicles at rail-highway grade crossings, they are not commonly used by highway engineers. The American Railway Engineering Association (AREA) grade crossing and ITE driveway design standards were evaluated with HANGUP using a vehicle with a 36-ft wheelbase and 5 in. of ground clearance. This can be considered as the standard or "design" low-clearance vehicle. On the basis of limited field data collection, such vehicles represented 85th-percentile values for ground clearance and wheelbase. The results indicate that the AREA design standards accommodate low-clearance vehicles but the ITE standards do not. Grade changes of more than 2.3 percent on each side of railroad grade crossings have the potential for causing low-clearance vehicles to become stuck. Grade changes at intersections should be less than or equal to 4.6 percent, which is the maximum slope rate for the standard low-clearance vehicle.

Only very limited data are available on the hang-up and overhang problems at railroad crossings and elsewhere on the highway system . Certain severe accidents have been publicized in the media and investigated by the National Transportation Safety Board (NTSB) (1,2). In telephone conversations, personnel from the Public Utility Commission of Oregon indicated that Oregon averages about one accident a year in which a low-clearance vehicle gets hung up on railroad tracks and is struck by a train. Discussions with local and state highway agency personnel and trucking company officials indicated that even though hard data are lacking, the problem is believed to be significant. For example, the mid-Atlantic region safety director for a trucking company that transports automobiles noted that his fleet experiences 50 to 60 hangup incidents per month. However, from accident data in general, it is usually not possible to identify which accidents are the result of low-clearance vehicles' becoming lodged on highprofile roadways. Thus, it is difficult to quantify the magnitude of the hang-up problem.

When a long wheelbase or low-ground-clearance vehicle negotiates a high-profile roadway-such as a railroad-highway grade crossing, roadway crown, or driveway entrance-the vehicle may become lodged or stuck on the "hump." There are several types of low-ground-clearance vehicle, as shown in Figure 1. These include lowboy equipment trailers, car carriers, single- and double-drop van trailers, and cars and trucks with trailers. A not-uncommon occurrence is one in which a railroad is on an embankment and a low-groundclearance vehicle on the crossing roadway becomes lodged on the track and is subsequently struck by a train. Hang-ups on railroad crossings are probably the most visible and dramatic of all incidents on high-profile roadways, but the problem also occurs relatively frequently at driveway entrances, street intersections, and roadway crowns.

Vehicle overhang is another cause of concern for low-groundclearance vehicles. On sag vertical curves, significant front and rear overhang may cause dragging. Even though overhang is a less significant situation than a hang-up, it creates many problems-including damage to the roadway surface, potential danger if the affected vehicle carries hazardous material, and delay and inconvenience to the truck and other motorists.

R. W. Eck, Department of Civil Engineering, West Virginia University, Morgantown, W.Va . 26506. S. K. Kang, Keimyung University, Shindong-dong, Dalsa-gu, Daegu, Korea.

FIGURE 1 Common types of low-clearance vehicles found in traffic stream.

Eck and Kang

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NTSB believes that although high-profile surfaces at grade crossings are not a statistically significant problem nationwide, the hazard is serious enough to warrant corrective measures (3). Countermeasures should be initiated relative to the identification of such crossings and the signing of crossings identified as hazardous to low-profile vehicles. Recently, NTSB recommended that FHWA identify design criteria to determine what geometric conditions on approaches to grade crossings would create a hazard to low-clearance vehicles and to develop geometric design criteria and traffic control systems for mitigating these hazards (3). A procedure to identify profile design criteria for rail-highway crossings and for highprofile roadway sections in general needs to be developed.

OBJECTIVES

Research was conducted to address the problem of lowclearance vehicles. Specific objectives of the study were

1. To determine, through a literature review, existing standards and guidelines aimed at accommodating low-groundclearance vehicles on high-profile roadways.

2. To develop a microcomputer software package to model the travel of low-clearance vehicles over a variety of highprofile geometries.

3. To apply the oftware package to evaluate the adequacy of existing standards and guidelines aimed at low-groundclearance vehicles.

4. To apply the software package to develop specific highway design criteria to accommodate low-clearance vehicles and to present them in a form suitable for inclusion in appropriate highway design standards.

The software package, HANGUP, has been described before (4,5). This paper will focus on the review and analysis of existing guidelines and on the development of design standards .

APPROACHES TO PROBLEM

The literature review indicated that researchers have been aware of vehicle ground-clearance problems for a number of years. However, efforts have been sporadic and directed at specific problems. There has been no integrated approach to address the ground-clearance problem in general.

As early as 1958, McConnell mentioned vehicle groundclearance problems in his review of 10-year trends (19481958) in domestic and foreign passenger car dimensions (6). Specific dimensions studied included wheelbase, angles of approach and departure, minimum ground-clearance, and ramp breakover angle. Figure 2 illustrates the concepts of angle of approach, angle of departure, and breakover angle for passenger cars.

McConnell concluded that the most critical condition was rear overhang on short-wheelbase vehicles under conditions of rear jounce, which is a vehicle's downward action in a sag vertical curve or bump (6). A sag vertical curve radius less than about 90 ft would bother short passenger cars. Long passenger cars would experience the same trouble on a sag

FIGURE 2 Concept of angle of approach (top), angle of departure (middle) and ramp breakover angle (bottom) (6).

radius of 80 ft or less. McConnell recommended that th~re be no more than a 5 percent change in slope betw ................
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