80 TRANSPORTATION RESEARCH RECORD 1356

Roadway Design Standards To Accommodate Low-Clearance Vehicles

RONALD w. ECK AND s. 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.

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.

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.

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

Eck and Kang

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 FHW A 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 (1948-1958) 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).

81

radius of 80 ft or less. McConnell recommended that th~re be no more than a 5 percent change in slope betw<;en any two 10-ft chords (that is, the ramp over a 6-in. curb should be at least 10 ft long) and that there be no more than 1.5 in. of clearance between the pavement and a 10-ft straightedge.

Bauer described the problem of insufficient ground clear­ance for automobiles traversing driveway entrances in sub­urban or residential areas (7). He proposed the use of a 2-in . safety margin for vehicle ground clearance to accommo­date the downward thrust that cars experience when brakes are applied while traversing the varying profile grade of driveway proposed. Figure 3 shows the driveway design propo ed by Bauer. Figure 3 (top) presents the profile for an ascending slope; Fi~ure 3 (bottom) shows that of a descending slope. When the driveway ascends steeply from the back of the walk into an owner's property, the ascent for the first 5 ft back of the walk should not be more than 10 in., or at the rate of 16 percent. For a descending slope, the descent should not be more than 2.5 in., or 4 percent, in the first 5 ft from the back edge of the walk and not more than 9 in., or 15 percent, in the next 5 ft, making the maximum permissible descent about 1 ft in the first 10 ft. The 25 percent grade shown on the illu tration is the maximum recommended grade for driveways on private property.

This was the precomputer era, so Bauer recommended a manual procedure for checking designs (7). The procedure involved cutting out a model car using a piece of cardboard at the same scale as the profile. The model could be slid along the profile to find any trouble spots, and the profile adjusted as necessary.

Given the early intere tin clearance problems at driveway and parking lot entrances, a portion of the .l iterature review focused on this topic. Vehicles entering and leaving driveways and traver ·ing parking .ramps face the possibility of hang-up or dragging of vehicle overhangs. The profile of the driveway or ramp is an important element because it affects potential damage to the unde1 ides of vehicles and the comfort of ve­hicle occupants. Similarly, an elevated pavement crown can

82

w u z c . m: I ~ I .J ' u l

HT

TRANSPORTATION RESEARCH RECORD 1356

!FT !HT VARIABLE

ORIVEWAY

CRITICAL CLE~ANCE

AREA

POSITION I

l[----+

4 .0 I".

3FT :SFT 5 FT

CUMI OAIVEWAY

--·-~

z ....

' ~ 1 .;»--~

FIGURE 3 Driveway profiles proposed by Bauer (7): ascending slope, top; descending slope, bottom.

cause a hang-up situation for !ow-clearance vehicles when they cross such roadways or when they attempt U-turns.

Standards established by SAE (8) for passenger cars and light-duty truck under full-rated load limit the ramp break­over angle and angle of departure to 10 degrees. The angle of approach should be no more than 16 degrees. Even though the angles for heavy truck were not mentioned in the SAE handbook the angles might be much higher than those of the pa senger cars and light-duty trucks. Veh.icles (pa . enger cars) designed to negotiate such geometry theoretically should be able to traverse sag and crest sections at the bottom and top of a 17.6 percent ramp grade and to move to flat grades without need for a grade transition area. However, a guideline for parking lot ramp suggests a minimum 12-ft-Jong transition slope equal to half of the ramp grade to increa e driver sight distance at the ramp crest and to give comfort at sag section (9) .

ITE bas published a guideline for driveways (JO). Figure 4 and Table 1 show desirable and suggested maximum grade changes for three classes of driveways. For the values shown in the table, no vertical curve connecting the tangents is nee-

essary.The value of G1 is limited by shoulder slope or by the presence of a sidewalk within the right-of-way but desirably il hould not exceed 10 percent. The maximum grade (G2)

generally should be Limited to 15 percent for residential drive­ways and 5 to 8 percent for commercial and iodu trial drive­ways.

The ITE guideline would work with the SAE vehicle stan­dards for passenger cars and light-duty trucks; however, it is doubtful that it could accommodate heavy trucks. The ITE guideline also suggests that for grade changes more abrupt than those shown in the figure, vertical curves at least 10 ft long should be used to connect tangents.

Low-ground-clearance vehicle hang-up accidents on railroad-highway grade crossings highlight the absence of readily available geometric standards for designing and maintaining roadway profiles at grade crossings. After it investigated a 1983 hang-up accident in North Carolina, NTSB (1) warned that crossing profiles with hump-like vertical curves can impede the operation of a vehicle if the distance between any two axles of a vehicle spans the hump and the height of the hump exceeds the vehicle's ground clearance. The report recom-

Eck and Kang

Edge of pavement or shoulder --....

I I I I I

83

Tangent

FIGURE 4 Driveway profiles suggested by ITE (JO).

mended that existing grade crossings with roadway profiles that may be hazardous to certain vehicles should be identified and, subsequently, improvements made. Identification of such crossings implies the need for a geometric design standard or criteria.

The America Railway Engineering Association (AREA) developed and published geometric design standards for railroad-highway grade crossings (11). The standards state, "It is desirable that the surface of the highway be not more than 3 inches higher nor 6 inches lower than the top of nearest rail at a point 30 feet from the rail, measured at right angle thereto, unless track superelevation dictates otherwise." Un­fortunately, there is no evidence that these guidelines were used in any of the accidents investigated by NTSB, most likely because the railway engineering document was not generally available to highway designers. It is interesting to note that the 1990 edition of the AASHTO Green Book included, for the first time, specific vertical alignment standards to accom­modate low-clearance vehicles (12). The standards presented are actually a restatement of the AREA standards.

RESULTS

Applications to Existing Design Standards

It seemed appropriate to apply the capabilities of the HANGUP software package to evaluate some of the low-clearance design standards currently in use. The AREA and ITE design stan-

TABLE 1 Suggested Grade Changes for Three Classes of Driveway Shown in Figure 4

Suggested Maximum Grade Change (D) (percent)

Type of Driveway Desirable Maximum

High volume 0 ±3 Low volume on

major or collector street ±3 ±6

Low volume on controlled by vehicle local street ±6 clearance ( ± 15)

dards were evaluated with HANGUP using a vehicle with a 36-ft wheelbase and 5 in. of ground clearance.

As noted, the AREA design standard was used in the 1990 edition of the AASHTO Green Book for grade crossing pro­files (12). As shown in Table 2, a 36-ft-wheelbase vehicle will not bang up until the ground clearance drops below 4 in. Therefore, the AREA design standard, as expected, was found to accommodate most low-clearance vehicles.

ITE's recommended driveway design, shown in Figure 4, was analyzed using HANGUP. The maximum grade change (D) of 3 percent was analyzed. The standard low-clearance vehicle will have problems with this design, as shown in Table 3. Consequently, most low-clearance vehicles will hang up on the recommended low-volume driveway profile. It can be concluded, therefore, that the ITE driveway design recom­mendations do not accommodate low-clearance vehicles. This should be pointed out clearly in the recommendations.

Development of Design Standards

Design Considerations

Field studies indicate that the trucks operating on the nation's highway system have a wide variety of wheelbases and ground clearances. Although it is clearly unusual, ground clearances as low as 2 in . have been found. Wheelbases, defined here as the distance from rear axle of the tractor to front axle of the trailer, are even more variable . Data from weigh stations indicate that wheelbases of low-ground-clearance vehicles can range from 21 to more than 40 ft.

It is not feasible to design the highway system to accom­modate such extremes. However, the system should accom­modate all of the common vehicle types. A logical choice, which has precedence in traffic engineering, would be to use 85th-percentile values for ground clearance and wheelbase . To meet this criterion vertical alignment should be de igned for vehicles with greater than or equal to 5 in. of ground clearance and for wheelbases of less than or equal to 36 ft. This can be considered as the design low-ground-clearance vehicle.

Different profile types may create problems for low-clearance vehicles. Three types of high-profile roadway are shown in

84 TRANSPORTATION RESEARCH RECORD 1356

TABLE 2 Output from Software Package HANGUP for AREA Design Standard Crossing Profile

Ground Clearance (in) --------------------------------------------------Wheel Base 1 2 3 4 5 6 7 8 9 10

--------------------------------------------------10 (ft) 0 0 0 0 0 0 0 0 0 0 11 (ft) 0 0 0 0 0 0 0 0 0 0 12 (ft) 0 0 0 0 0 0 0 0 0 0 13 (ft) 0 0 0 0 0 0 0 0 0 0 14 (ft) 1 0 0 0 0 0 0 0 0 0 15 (ft) 1 0 0 0 0 0 0 0 0 0 16 (ft) 1 0 0 0 0 0 0 0 0 0 17 (ft) 1 0 0 0 0 0 0 0 0 0 18 (ft) 1 0 0 0 0 0 0 0 0 0 19 (ft) 1 0 0 0 0 0 0 0 0 0 20 (ft) 1 0 0 0 0 0 0 0 0 0 21 (ft) 1 0 0 0 0 0 0 0 0 0 22 (ft) 1 0 0 0 0 0 0 0 0 0 23 (ft) 1 0 0 0 0 0 0 0 0 0 24 (ft) 1 1 0 0 0 0 0 0 0 0 25 (ft) 1 1 0 0 0 0 0 0 0 0 26 (ft) 1 1 0 0 0 0 0 0 0 0 27 (ft) 1 1 0 0 0 0 0 0 0 0 28 (ft) 1 1 0 0 0 0 0 0 0 0 29 (ft) 1 1 0 0 0 0 0 0 0 0 30 (ft) 1 1 0 0 0 0 0 0 0 0 31 (ft) 1 1 0 0 0 0 0 0 0 0 32 (ft) 1 1 0 0 0 0 0 0 0 0 33 (ft) 1 1 0 0 0 0 0 0 0 0 34 (ft) 1 l 1 0 0 0 0 0 0 0 35 (ft) 1 1 1 0 0 0 0 0 0 0 36 (ft) 1 1 1 0 0 0 0 0 0 0 37 (ft) l l 1 0 0 0 0 0 0 0 38 (ft) 1 1 1 0 0 0 0 0 0 0 39 (ft) 1 1 1 0 0 0 0 0 0 0 40 (ft) 1 1 1 0 0 0 0 0 0 0

--------------------------------------------------l -> Hanq up,

Figure 5. The first two profiles have the potential to cause hang-ups for low-clearance vehicles . The last profile can cause dragging or scraping problems for vehicles with long over­hangs. Table 4 presents the maximum grades and elevation differences at a point 30 ft from outer rails for the Type I profile. Table 5 presents the minimum crest vertical curve lengths for Type II profiles that can safely accommodate the design low-clearance vehicle.

The design and construction of crest vertical curves for low­ground-clearance vehicles involve several considerations:

1. To eliminate hang-up incidents at high-profile roadways, the rate of change of grade on crest vertical curves should be constant, that is, a parabolic curve.

2. All Interstate and primary highways should accommo­date the design low-ground-clearance vehicle .

3. All secondary highways and local roads that cannot meet the design standard should provide an advance warning sign advising of the potential clearance problem.

4. A detour route or turning space on a relatively level area,

O -> Safe

which does not interrupt through traffic, must also be pro­vided for low-ground-clearance vehicles in advance of the crossing.

5. Weigh stations should measure wheelbase and ground clearance under static conditions for vehicles whose ability to negotiate high-profile roadways appears questionable .

6. On existing roadway profiles, surface maintenance is very important. Pavement patches or pavement defects can lead to hang-ups for certain low-ground-clearance vehicles.

Prevention of Overhang Dragging

When a low-ground-clearance vehicle traverses a sag vertical curve, significant front and rear overhang of the vehicle may cause dragging. Figure 6 represents the rear end of a long­overhang vehicle, where V is the rear-overhang length (in feet) and his the ground-clearance (in inches) in a sag vertical curve. Equation 1 gives the minimum sag vertical curve length to prevent the overhang problem.

Eck and Kang 85

TABLE 3 Output from Software Package HANGUP for Profile Recommended by ITE for High-Volume Driveways

Ground Clearance (in) --------------------------------------------------Wheel Base l 2 3 4 5 6 7 8 9 10

--------------------------------------------------10 (ft) l 0 0 0 0 0 0 0 0 0 11 (ft) 1 0 0 0 0 0 0 0 0 0 12 (ft) 1 l 0 0 0 0 0 0 0 0 13 (ft) 1 l 0 0 0 0 0 0 0 0 14 (ft) l l 0 0 0 0 0 0 0 0 15 (ft) 1 l 0 0 0 0 0 0 0 0 16 (ft) l l 0 0 0 0 0 0 0 0 17 (ft) 1 l l 0 0 0 0 0 0 0 18 (ft) 1 l l 0 0 0 0 0 0 0 19 (ft) 1 l 1 0 0 0 0 0 0 0 20 (ft) l l l 0 0 0 0 0 0 0 21 (ft) l l l 0 0 0 0 0 0 0 22 (ft) l l l 0 0 0 0 0 0 0 23 (ft) l l l l 0 0 0 0 0 0 24 (ft) l l l l 0 0 0 0 0 0 25 (ft) l 1 1 l 0 0 0 0 0 0 26 (ft) 1 1 1 l 0 0 0 0 0 0 27 (ft) l l 1 l 0 0 0 0 0 0 28 (ft) 1 1 l 1 l 0 0 0 0 0 29 (ft) l 1 1 1 1 0 0 0 0 0 30 (ft) 1 1 l 1 1 0 0 0 0 0 31 (ft) 1 1 1 1 1 0 0 0 0 0 32 (ft) 1 1 1 1 1 0 0 0 0 0 33 (ft) 1 1 1 1 l 0 0 0 0 0 34 (ft) 1 1 1 1 1 1 0 0 0 0 35 (ft) 1 l l 1 l 1 0 0 0 0 36 (ft) 1 l l 1 l 1 0 0 0 0 37 (ft) 1 1 1 1 1 1 0 0 0 0 38 (ft) 1 1 1 l 1 1 0 0 0 0 39 (ft) 1 1 l l 1 1 1 0 0 0 40 (ft) 1 1 1 1 1 1 1 0 0 0

--------------------------------------------------1 -> Hang up, o -> Sate

L . = l 44 V2 + h cos (90 - Y2 tan - 1(A)] (1) I- w -j ~----~+g~------~~----~---g--~

mm 12/i

2 A = g2 - gl

-g I- w -1 +

FIGURE 5 Three types of high-profile roadway: Type I, top; Type II, middle; Type III, bottom.

where

L = length of sag vertical curve (ft), V = overhang length of the vehicle (ft), h = ground clearance of the vehicle (in .), and A = algebraic difference in grades (percent/100).

Table 6 presents the minimum lengths of sag vertical curves to prevent overhang dragging for the design low-clearance vehicle. For example, when a low-ground-clearance vehicle whose rear overhang length is 5 ft and ground clearance is 5 in. traverses a sag vertical curve wi th a 10 percent algebraic difference in grade the minimum length of the sag curve should be 3 ft. For overall safety on highway , sag vertica l curves should be long enough so that low-ground-clearance vehicles can traverse the curve without dragging.

TABLE 4 Maximum Grades and Elevation Differences for Type I Profile To Accommodate the Design Low­Clearance Vehicle

Width of Level Maximum Safe Max. Eleva. Dif-Section, w (ft) Grades, g (\) ferences, h (in)

4 1 8. 5 11 2.5 9.0

6 2.7 9.5

8 2.9 9.7

10 3.1 10.2

12 3.3 10.4

14 3.5 10.6

16 3.8 11.1

18 4.1 11. 5

20 4.4 11.8

22 or more 4.6 11.8

TABLES Minimum Crest Vertical Curve Lengths for Type II Profiles To Accommodate Design Low­Clearance Vehicle

Algebraic Difference, A (\) curve Length, L (ft)

1 4

2 8

3 12

4 16

5 20

6 24

7 28

8 32

9 35

10 39

Parabolic Profile

FIGURE 6 Rear end of long overhang vehicle with sag vertical curve.

Eck and Kang 87

TABLE 6 Minimum Lengths of Sag Vertical Curves To Prevent Overhang Dragging for Design Low-Clearance Vehicle

CLEARANCE (IN. )

1 2 3 4 5 6 7 8 9

1 0.6 0.3 0.2 0.2 0.1 0.1 0.1 0.1 0.1

2 2.4 1.2 0.8 0.6 0.5 0.4 0.4 0.3 0.3

3 5.4 2.7 1. 8 1.4 1.1 0.9 0.8 0.7 0.6

4 9.6 4.8 3.2 2.4 1. 9 1. 6 1. 4 1. 2 1.1

5 14.9 7.5 5.0 3.8 3.0 2.5 2.2 1.9 1. 7

6 21. 5 10.8 7.2 5.4 4.3 3.6 3.1 2.7 2.4

7 29.3 14.7 9.8 7.3 5.9 4.9 4.2 3.7 3.3

8 38.3 19.1 12.8 9.6 7.7 6.4 5.5 4.8 4.3

9 48.4 24.2 16.2 12.1 9.7 8.1 6.9 6.1 5.4

10 59.8 29.9 19.9 15.0 12 . 0 10.0 8.6 7.5 6.7

Note Grade Difference = 10 t, Unit for lengths is in . ft.

CONCLUSIONS AND RECOMMENDATIONS

This study has attempted to use computer software to develop geometric design standards to accommodate low-ground­clearance vehicles. This objective was achieved through the development 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 low-clearance vehicles at crossings, they are not generally known to highway designers and, therefore, are not used. Apparently, the standards have been based on expe­rience or on a seat-of-the-pants approach rather than on a formal analysis of crossing geometry and truck characteristics. No evaluation of these standards has been made to assess their adequacy. The 1990 edition of the AASHTO Green Book incorporates the AREA profile for railroad-highway grade crossings; however, the high-profile problem on high­ways in general is not addressed.

Grade changes of more than 2.3 percent on each side of the crossing leave the potential for low-clearance vehicles to become stuck. From the results of HANGUP, a few crossing profiles that would accommodate low-clearance vehicles were suggested. To resolve the overhang dragging problem, an equation and a table were developed to give minimum lengths of sag vertical curves.

Grade changes at inter ection are al o an important design factor for highway engiDeers . ff two roadways inter. ect, grade difference at the intersection should be less than or equal to 4.6 percent, which is the maximum slope rate for the standard low-clearance vehicle.

It is apparent that highway and traffic engineers need to pay more attention to the ground-clearance problem in gen­eral, at rail-highway grade crossings in particular. Attention in this sense also includes the matter of communicating ex­isting design standards to practitioners.

Additional wheelbase and ground-clearance data need to be collected from several geographic regions before specific dimensions can be specified for each of the low-clearance vehicle categories.

Low-clearance vehicles also need to be considered in high· way operations. For example, the current permitting process for over ize vehicles con iders weight, height, and width but apparently not ground clearance. This oversight should be corrected. The wide variety in dimensions of low-clearance vehicles should be examined with an eye toward po ibly es­tablishing reasonable minimum ground-clearance tandards for vehicles Operating on public highways. This is especially important in light of the growing variety of specialized vel1icles in the traffic stream.

ACKNOWLEDGMENTS

Support for the re earch de cribed in thi · paper wa provided by a U.S. Department of Transportation/Univer'ity Trans­portation Centers Program grant and by the Division of High­ways West Virginia Department of Transportation (WVDOT). The encouragement and cooperation provided throughout the project by Ray Lewi of WVDOT and Richard Mather of the Public Utility Commission of Oregon are sincerely appreciated.

REFERENCES

l. Railroad/Highway Accident Report- Co//isio11 o.f Amtrak Traill No. 88 ivitlr Tractor Lowboy Semitrailer Combi11atio11 Tmck , Rowland North Carolina Augus125, 1983 Report NTSBfRHR-84/01. National Transportation afety Board. Wasl1ington, D.C .. Aug. 1984.

88

2. Safety Study-Pa.vscnger!Commwer Trai11 and Motor Vehicle ollisions at Grade Crossi11gs (1985). Report NTSD/S -86104.

National Transportation Safety Board, Washington, D .C., Dec. 1986.

3. Safl!ly Recom111e11d111io11s H· 9-6 a11d 11-89-7. National Trans­portation Safety Board, Wa hington , D. . Feb. 19 9.

4. S. K. Kang and R. W. Eck . Low-Ground-Ocarance Vehicles at Railroad-Highway 01 mJc Ci o. lngs: A Design and Analysis Soft­ware Package. Proc., /111ema1io11a/ Symposium 011 Railroad· Hif:l11vay 1rade Crossing Research and Safety , University orTcn­nessee, Knoxv ille, Nov. 1990 (in press).

S. R. W. Eck and S. K. Kang. Low-Chrnrnnce Vehicles at Rail · Highway Grade Crossings: An Overview or the Problem and Potential Solutions . In Tm11 portation Rese11rclt Recortl 1327, TRB, National Research Council, Washington, D.C., 1991.

6. W. A. McConnell. Passenger Car Overhang and Underclearance as Related to Driveway Profile Design, Part I-Vehicle Data. Bulletin 195, HRB, National Research Council, Washington, D.C., 1958, pp. 14- 23 .

7. L. A. Bauer. Passenge r ar vcrhang and Undcrclearance as Related to Driveway Profile De ign, Part 11'-Strcct a11d High­way Design. Balle1i11 195, HRB , National Rcse<irch uncil, Washington , D.C., 1958, pp. 23-29.

8. 1988 SAE H1111d/Jook, Vol. 4. SAE, Warrendale, Pa., 1988. 9. R. A. Weant. Parking Garage Planning and Operation. Eno

Foundation for Transportation, Inc., Westport, 01rn . , 1978. 10. Guidelinl!s for Driveway Location and Design. Technical Com­

mittee SB-13, ITE, Washington, D.C., 1987. 11. Manual for Railway Engineering. American Railway Engineering

Association. Chicag , III. , 1971. 12. A Policy on Geometric Design of Highways and Streets. AASHTO,

Washington, D.C., 1990.

The contents of tire paper reflect tlie views of the awlrors, who are responsible for the facts arul ac 11r11cy of the informlllio11 prese111ed herein.

DISCUSSION

J. L. GATTIS University of Oklahoma, 202 West Boyd, Room 334, Norman, Okla. 73019.

As one who has been a practicing roadway design engineer, I believe that Eck and Kang have provided useful information in this article. Authors of roadway and driveway design pub­lications should include a similar development of these issues .

In the "Developing Design Standards" section of the paper, the authors discuss using short vertical curves to accommodate low-clearance vehicles. Before selecting a short vertical curve design, an engineer should consider not only the impacts of future surface maintenance (as the authors recommended), but also the tolerances to which the field crews will build a short vertical curve. It may be that without very close super­vision, the field crews will actually build short tangents instead of the intended vertical curves.

In the same section, the authors note that ground clearances as low as 2 in. have been found; because such values have "precedence in traffic engineering," they favor 85th-percentile values for ground clearance and wheelbase design. I believe that the use of the 85th-percentile value in establishing speed limits or other traffic engineering criteria to which the authors were alluding does not necessarily justify its use for underside­clearance design. An alternative approach for arriving at the proper percentile for design would involve considering the attributes of the particular issue at hand, in this case the

TRANSPORTATION RESEARCH RECORD 1356

distribution of vehicle ground-clearance frequencies. The dis­tribution of clearance dimensions may be such that only the least 5 percent (or some other value) are outliers. Even then, issues such as the consequences of not accommodating a cer­tain percentage, or the possibility that certain classes of out­liers may always be found at certain types of land uses (e.g., automobile dealerships) should be considered.

One would not expect vehicles with extremely low clear­ances to enter driveways serving many types of land uses or local residential streets. If vehicles with extremely low clear­ance are more likely to enter a commercial site or an arterial street than a residential site or a local street, then perhaps there should be a number of low-clearance design vehicles, each of them appropriate for a particular application.

The authors allude to the underlying problem in the final section, when they consider the possibility of establishing ground-clearance standards for vehicles on the public high­ways. If the designer is to consider the driver, the vehicle, and the roadway as components of an engineered system, then manufacturers must build vehicles that conform to di­mensional requirements such as minimum underside clear­ance and minimum eye-height standards. It seems illogical to charge street and highway officials with the task of designing roadways that will be safe for years to come when the critical attributes of vehicles on the roadway are not controlled. With­out such controls, designers will either be chasing an ever­moving target in an attempt to define values for those attributes that govern design or be selecting costly, overly conservative design values in an attempt to address any imaginable vehicle· design situation.

AUTHORS' CLOSURE

We greatly appreciate the thoughtful and constructive review of our paper by Gattis. We recognized early on that the design criteria proposed in the paper were preliminary in nature because they were based on limited data. Thus, one of our objectives in preparing this paper was to seek input from practitioners on the reasonableness of our proposed criteria and to stimulate interest in collecting additional vehicle data nationally.

We agree completely with the comment about tolerances attainable in the field and agree that this is an area that needs attention. In our study, we observed a number of locations where the initial design was sound but construction or main· tenance operations changed the surface of the road such that there was a potential for hangups.

We concur with the discussant's remarks about use of the 85th-percentile value. We proposed this as a starting point for discussions because it did have some precedence in traffic engineering practice. Certainly, the value or values chosen here should reflect a balance between the economic realities faced by highway agencies and the needs of the trucking in­dustry. Open discussion of this issue would be desirable, in our view.

The suggestion about developing several low-clearance de­sign vehicles has considerable merit. However, there are not sufficient data to accomplish this now. As noted in our rec·

Eck and Kang

ommendations, we believe that it is important that additional data on the physical characteristics of low-clearance vehicles be collected from all regions of the country. Such information would be useful in establishing the vehicle dimensional re­quirements proposed by the discussant.

As the discussant clearly points out, there is still consid­erable work to be done before criteria to accommodate low-

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clearance vehicles become a formal part of any design policy. We hope that discu sion relative to this topic will continue to take place. Interested readers are encouraged to respond formally or informally to R. W. Eck.

Publication of this paper sponsored by Committee on Geometric Design.