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 Only very limited data are available on the hang-up and accommodate low-ground-clearance vehicles using computer soft overhang problems at railroad crossings and elsewhere on the ware. Low-clearance vehicles include lowboy equipment trailers, highway system. Certain severe accidents have been publi car carriers, single- and double-drop van trailers, and cars and cized in the media and investigated by the National Trans trucks with trailers. Hang-ups and overhang dragging on high profile roadways are causes of concern for low-ground-clearance portation Safety Board (NTSB) (1,2). In telephone conver vehicles. The objective was achieved through the development sations, personnel from the Public Utility Commission of and application of the HANGUP software package and the anal Oregon indicated that Oregon averages about one accident a ysis of the design standards of several agencies. Although a few year in which a low-clearance vehicle gets hung up on railroad agencies have developed geometric design standards for low tracks and is struck by a train. Discussions with local and state clearance vehicles at rail-highway grade crossings, they are not highway agency personnel and trucking company officials in commonly used by highway engineers. The American Railway Engineering Association (AREA) grade crossing and ITE drive dicated that even though hard data are lacking, the problem way design standards were evaluated with HANGUP using a is believed to be significant. For example, the mid-Atlantic vehicle with a 36-ft wheelbase and 5 in. of ground clearance. This region safety director for a trucking company that transports can be considered as the standard or "design" low-clearance ve automobiles noted that his fleet experiences 50 to 60 hang hicle. On the basis of limited field data collection, such vehicles up incidents per month. However, from accident data in gen represented 85th-percentile values for ground clearance and eral, it is usually not possible to identify which accidents are wheelbase. The results indicate that the AREA design standards accommodate low-clearance vehicles but the ITE standards do the result of low-clearance vehicles' becoming lodged on high not. Grade changes of more than 2.3 percent on each side of profile roadways. Thus, it is difficult to quantify the magnitude railroad grade crossings have the potential for causing low-clearance of the hang-up problem. 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 ne gotiates 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-ground clearance 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 in tersections, and roadway crowns. Vehicle overhang is another cause of concern for low-ground clearance 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, poten tial 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 Uni versity, Morgantown, W.Va . 26506. S. K. Kang, Keimyung Univer FIGURE 1 Common types of low-clearance vehicles found in sity, Shindong-dong, Dalsa-gu, Daegu, Korea. traffic stream. Eck and Kang 81
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 iden tification of such crossings and the signing of crossings iden tified as hazardous to low-profile vehicles. Recently, NTSB recommended that FHWA identify design criteria to deter mine what geometric conditions on approaches to grade cross ings 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 pro file design criteria for rail-highway crossings and for high profile roadway sections in general needs to be developed.
OBJECTIVES
Research was conducted to address the problem of low clearance vehicles. Specific objectives of the study were FIGURE 2 Concept of angle of approach (top), angle of departure (middle) and ramp breakover angle (bottom) 1. To determine, through a literature review, existing stan (6). dards and guidelines aimed at accommodating low-ground clearance vehicles on high-profile roadways. 2. To develop a microcomputer software package to model radius of 80 ft or less. McConnell recommended that th~re the travel of low-clearance vehicles over a variety of high be no more than a 5 percent change in slope betw<;en any two profile geometries. 10-ft chords (that is, the ramp over a 6-in. curb should be at 3. To apply the oftware package to evaluate the adequacy least 10 ft long) and that there be no more than 1.5 in. of of existing standards and guidelines aimed at low-ground clearance between the pavement and a 10-ft straightedge. clearance vehicles. Bauer described the problem of insufficient ground clear 4. To apply the software package to develop specific high ance for automobiles traversing driveway entrances in sub way design criteria to accommodate low-clearance vehicles urban or residential areas (7). He proposed the use of a 2- and to present them in a form suitable for inclusion in ap in . safety margin for vehicle ground clearance to accommo propriate highway design standards. date the downward thrust that cars experience when brakes are applied while traversing the varying profile grade of The software package, HANGUP, has been described before driveway proposed. Figure 3 shows the driveway design (4 ,5). This paper will focus on the review and analysis of propo ed by Bauer. Figure 3 (top) presents the profile for an existing guidelines and on the development of design stan ascending slope; Fi~ure 3 (bottom) shows that of a descending dards. 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 APPROACHES TO PROBLEM 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 literature review indicated that researchers have been the back edge of the walk and not more than 9 in., or 15 aware of vehicle ground-clearance problems for a number of percent, in the next 5 ft, making the maximum permissible years. However, efforts have been sporadic and directed at descent about 1 ft in the first 10 ft. The 25 percent grade specific problems. There has been no integrated approach to shown on the illu tration is the maximum recommended grade address the ground-clearance problem in general. for driveways on private property. As early as 1958, McConnell mentioned vehicle ground This was the precomputer era, so Bauer recommended a clearance problems in his review of 10-year trends (1948- manual procedure for checking designs (7). The procedure 1958) in domestic and foreign passenger car dimensions (6). involved cutting out a model car using a piece of cardboard Specific dimensions studied included wheelbase, angles of ap at the same scale as the profile. The model could be slid along proach and departure, minimum ground-clearance, and ramp the profile to find any trouble spots, and the profile adjusted breakover angle. Figure 2 illustrates the concepts of angle of as necessary. approach, angle of departure, and breakover angle for pas Given the early intere tin clearance problems at driveway senger cars. and parking lot entrances, a portion of the .l iterature review McConnell concluded that the most critical condition was focused on this topic. Vehicles entering and leaving driveways rear overhang on short-wheelbase vehicles under conditions and traver ·ing parking .ramps face the possibility of hang-up of rear jounce, which is a vehicle's downward action in a sag or dragging of vehicle overhangs. The profile of the driveway vertical curve or bump (6). A sag vertical curve radius less or ramp is an important element because it affects potential than about 90 ft would bother short passenger cars. Long damage to the unde1 ides of vehicles and the comfort of ve passenger cars would experience the same trouble on a sag hicle occupants. Similarly, an elevated pavement crown can 82 TRANSPORTATION RESEARCH RECORD 1356
w uz c . m: I ~ I .J' u l
HT !FT !HT VARIABLE
ORIVEWAY
CRITICAL CLE~ANCE AREA l[------· -~ .... POSITION I z ~ + '1 4 .0 I". .;»--~
3FT :SFT 5 FT
CUMI OAIVEWAY
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 essary.The value of G 1 is limited by shoulder slope or by the cross such roadways or when they attempt U-turns. presence of a sidewalk within the right-of-way but desirably Standards established by SAE (8) for passenger cars and il hould not exceed 10 percent. The maximum grade (G2) light-duty truck under full-rated load limit the ramp break generally should be Limited to 15 percent for residential drive over angle and angle of departure to 10 degrees. The angle ways and 5 to 8 percent for commercial and iodu trial drive of approach should be no more than 16 degrees. Even though ways. the angles for heavy truck were not mentioned in the SAE The ITE guideline would work with the SAE vehicle stan handbook the angles might be much higher than those of the dards for passenger cars and light-duty trucks; however, it is pa senger cars and light-duty trucks. Veh.icles (pa .enger cars) doubtful that it could accommodate heavy trucks. The ITE designed to negotiate such geometry theoretically should be guideline also suggests that for grade changes more abrupt able to traverse sag and crest sections at the bottom and top than those shown in the figure, vertical curves at least 10 ft of a 17.6 percent ramp grade and to move to flat grades long should be used to connect tangents. without need for a grade transition area. However, a guideline Low-ground-clearance vehicle hang-up accidents on for parking lot ramp suggests a minimum 12-ft-Jong transition railroad-highway grade crossings highlight the absence of readily slope equal to half of the ramp grade to increa e driver sight available geometric standards for designing and maintaining distance at the ramp crest and to give comfort at sag section roadway profiles at grade crossings. After it investigated a (9) . 1983 hang-up accident in North Carolina, NTSB (1) warned ITE bas published a guideline for driveways (JO). Figure 4 that crossing profiles with hump-like vertical curves can impede and Table 1 show desirable and suggested maximum grade the operation of a vehicle if the distance between any two changes for three classes of driveways. For the values shown axles of a vehicle spans the hump and the height of the hump in the table, no vertical curve connecting the tangents is nee- exceeds the vehicle's ground clearance. The report recom- Eck and Kang 83
Edge of pavement or shoulder --.... I I I I I
Tangent
FIGURE 4 Driveway profiles suggested by ITE (JO). mended that existing grade crossings with roadway profiles dards were evaluated with HANGUP using a vehicle with a that may be hazardous to certain vehicles should be identified 36-ft wheelbase and 5 in. of ground clearance. and, subsequently, improvements made. Identification of such As noted, the AREA design standard was used in the 1990 crossings implies the need for a geometric design standard or edition of the AASHTO Green Book for grade crossing pro criteria. files (12). As shown in Table 2, a 36-ft-wheelbase vehicle will The America Railway Engineering Association (AREA) not bang up until the ground clearance drops below 4 in. developed and published geometric design standards for Therefore, the AREA design standard, as expected, was found railroad-highway grade crossings (11). The standards state, to accommodate most low-clearance vehicles. "It is desirable that the surface of the highway be not more ITE's recommended driveway design, shown in Figure 4, than 3 inches higher nor 6 inches lower than the top of nearest was analyzed using HANGUP. The maximum grade change rail at a point 30 feet from the rail, measured at right angle (D) of 3 percent was analyzed. The standard low-clearance thereto, unless track superelevation dictates otherwise." Un vehicle will have problems with this design, as shown in Table fortunately, there is no evidence that these guidelines were 3. Consequently, most low-clearance vehicles will hang up on used in any of the accidents investigated by NTSB, most likely the recommended low-volume driveway profile. It can be because the railway engineering document was not generally concluded, therefore, that the ITE driveway design recom available to highway designers. It is interesting to note that mendations do not accommodate low-clearance vehicles. This the 1990 edition of the AASHTO Green Book included, for should be pointed out clearly in the recommendations. the first time, specific vertical alignment standards to accom modate low-clearance vehicles (12). The standards presented are actually a restatement of the AREA standards. Development of Design Standards
Design Considerations RESULTS Field studies indicate that the trucks operating on the nation's Applications to Existing Design Standards highway system have a wide variety of wheelbases and ground clearances. Although it is clearly unusual, ground clearances It seemed appropriate to apply the capabilities of the HANGUP as low as 2 in . have been found. Wheelbases, defined here software package to evaluate some of the low-clearance design as the distance from rear axle of the tractor to front axle of standards currently in use. The AREA and ITE design stan- 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 accom TABLE 1 Suggested Grade Changes for Three Classes of modate such extremes. However, the system should accom Driveway Shown in Figure 4 modate all of the common vehicle types. A logical choice, Suggested Maximum which has precedence in traffic engineering, would be to use Grade Change (D) (percent) 85th-percentile values for ground clearance and wheelbase. Type of Driveway Desirable Maximum To meet this criterion vertical alignment should be de igned High volume 0 ±3 for vehicles with greater than or equal to 5 in. of ground Low volume on clearance and for wheelbases of less than or equal to 36 ft. major or collector This can be considered as the design low-ground-clearance street ±3 ±6 vehicle. Low volume on controlled by vehicle Different profile types may create problems for low-clearance local street ±6 clearance ( ± 15) 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, O -> Safe
Figure 5. The first two profiles have the potential to cause which does not interrupt through traffic, must also be pro hang-ups for low-clearance vehicles. The last profile can cause vided for low-ground-clearance vehicles in advance of the dragging or scraping problems for vehicles with long over crossing. hangs. Table 4 presents the maximum grades and elevation 5. Weigh stations should measure wheelbase and ground differences at a point 30 ft from outer rails for the Type I clearance under static conditions for vehicles whose ability to profile. Table 5 presents the minimum crest vertical curve negotiate high-profile roadways appears questionable. lengths for Type II profiles that can safely accommodate the 6. On existing roadway profiles, surface maintenance is very design low-clearance vehicle. important. Pavement patches or pavement defects can lead The design and construction of crest vertical curves for low to hang-ups for certain low-ground-clearance vehicles. ground-clearance vehicles involve several considerations:
1. To eliminate hang-up incidents at high-profile roadways, Prevention of Overhang Dragging the rate of change of grade on crest vertical curves should be constant, that is, a parabolic curve. When a low-ground-clearance vehicle traverses a sag vertical 2. All Interstate and primary highways should accommo curve, significant front and rear overhang of the vehicle may date the design low-ground-clearance vehicle. cause dragging. Figure 6 represents the rear end of a long 3. All secondary highways and local roads that cannot meet overhang vehicle, where V is the rear-overhang length (in the design standard should provide an advance warning sign feet) and his the ground-clearance (in inches) in a sag vertical advising of the potential clearance problem. curve. Equation 1 gives the minimum sag vertical curve length 4. A detour route or turning space on a relatively level area, 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
44 L . = l V2 + h cos (90 - Y2 tan - 1(A)] (1) I- w -j mm 12/i ~----~+g~------~~----~---g --~ where
L = length of sag vertical curve (ft), V = overhang length of the vehicle (ft), 2 h = ground clearance of the vehicle (in.), and A = g2 - gl 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 -g I- w -1 + difference in grade the minimum length of the sag curve should be 3 ft. For overall safety on hi ghway , sag vertical FIGURE 5 Three types of high-profile roadway: Type I, top; curves should be long enough so that low-ground-clearance Type II, middle; Type III, bottom. vehicles can traverse the curve without dragging. TABLE 4 Maximum Grades and Elevation Differences for Type I Profile To Accommodate the Design Low Clearance 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 Low Clearance 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 Additional wheelbase and ground-clearance data need to be collected from several geographic regions before specific This study has attempted to use computer software to develop dimensions can be specified for each of the low-clearance geometric design standards to accommodate low-ground vehicle categories. clearance vehicles. This objective was achieved through the Low-clearance vehicles also need to be considered in high· development of the HANGUP software package and the anal way operations. For example, the current permitting process ysis of the design standards of several agencies. for over ize vehicles con iders weight, height, and width but Although a few agencies have developed geometric design apparently not ground clearance. This oversight should be standards for low-clearance vehicles at crossings, they are not corrected. The wide variety in dimensions of low-clearance generally known to highway designers and, therefore, are not vehicles should be examined with an eye toward po ibly es used. Apparently, the standards have been based on expe tablishing reasonable minimum ground-clearance tandards rience or on a seat-of-the-pants approach rather than on a for vehicles Operating on public highways. This is especially formal analysis of crossing geometry and truck characteristics. important in light of the growing variety of specialized vel1icles No evaluation of these standards has been made to assess in the traffic stream. 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 high ACKNOWLEDGMENTS ways in general is not addressed. Grade changes of more than 2.3 percent on each side of Support for the re earch de cribed in thi · paper wa provided the crossing leave the potential for low-clearance vehicles to by a U.S. Department of Transportation/Univer'ity Trans become stuck. From the results of HANGUP, a few crossing portation Centers Program grant and by the Division of High profiles that would accommodate low-clearance vehicles were ways West Virginia Department of Transportation (WVDOT). suggested. To resolve the overhang dragging problem, an The encouragement and cooperation provided throughout the equation and a table were developed to give minimum lengths project by Ray Lewi of WVDOT and Richard Mather of of sag vertical curves. the Public Utility Commission of Oregon are sincerely Grade changes at inter ection are al an important design o appreciated. 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. REFERENCES It is apparent that highway and traffic engineers need to pay more attention to the ground-clearance problem in gen l. Railroad/Highway Accident Report- Co//isio11 o.f Amtrak Traill No. 88 ivitlr Tractor Lowboy Semitrailer Combi11atio11 Tmck, eral, at rail-highway grade crossings in particular. Attention Rowland North Carolina Augus125, 1983 Report NTSBfRHR- in this sense also includes the matter of communicating ex 84/01. National Transportation afety Board. Wasl1ington, D.C.. isting design standards to practitioners. Aug. 1984. 88 TRANSPORTATION RESEARCH RECORD 1356
2. Safety Study-Pa.vscnger!Commwer Trai11 and Motor Vehicle distribution of vehicle ground-clearance frequencies. The dis ollisions at Grade Crossi11gs (1985). Report NTSD/S -86104. tribution of clearance dimensions may be such that only the National Transportation Safety Board, Washington, D .C., Dec. least 5 percent (or some other value) are outliers. Even then, 1986. 3. Safl!ly Recom111e11d111io11s H· 9-6 a11d 11-89-7. National Trans issues such as the consequences of not accommodating a cer portation Safety Board, Wa hington , D. . Feb. 19 9. tain percentage, or the possibility that certain classes of out 4. S. K. Kang and R. W. Eck. Low-Ground-Ocarance Vehicles at liers may always be found at certain types of land uses (e.g., Railroad-Highway 01 mJc Ci o. lngs: A Design and Analysis Soft automobile dealerships) should be considered. ware Package. Proc., /111ema1io11a/ Symposium 011 Railroad· Hif:l11vay 1rade Crossing Research and Safety, University orTcn One would not expect vehicles with extremely low clear nessee, Knoxville, Nov. 1990 (in press). ances to enter driveways serving many types of land uses or S. R. W. Eck and S. K. Kang. Low-Chrnrnnce Vehicles at Rail · local residential streets. If vehicles with extremely low clear Highway Grade Crossings: An Overview or the Problem and ance are more likely to enter a commercial site or an arterial Potential Solutions. In Tm11 portation Rese11rclt Recortl 1327, TRB, street than a residential site or a local street, then perhaps National Research Council, Washington, D.C., 1991. 6. W. A. McConnell. Passenger Car Overhang and Underclearance there should be a number of low-clearance design vehicles, as Related to Driveway Profile Design, Part I-Vehicle Data. each of them appropriate for a particular application. Bulletin 195, HRB, National Research Council, Washington, D.C., The authors allude to the underlying problem in the final 1958, pp. 14- 23 . section, when they consider the possibility of establishing 7. L. A. Bauer. Passenge r ar vcrhang and Undcrclearance as Related to Driveway Profile De ign, Part 11'-Strcct a11d High ground-clearance standards for vehicles on the public high way Design. Balle1i11 195, HRB, National Rcse
DISCUSSION AUTHORS' CLOSURE J. L. GATTIS University of Oklahoma, 202 West Boyd, Room 334, Norman, We greatly appreciate the thoughtful and constructive review Okla. 73019. of our paper by Gattis. We recognized early on that the design As one who has been a practicing roadway design engineer, criteria proposed in the paper were preliminary in nature I believe that Eck and Kang have provided useful information because they were based on limited data. Thus, one of our in this article. Authors of roadway and driveway design pub objectives in preparing this paper was to seek input from lications should include a similar development of these issues. practitioners on the reasonableness of our proposed criteria In the "Developing Design Standards" section of the paper, and to stimulate interest in collecting additional vehicle data the authors discuss using short vertical curves to accommodate nationally. low-clearance vehicles. Before selecting a short vertical curve We agree completely with the comment about tolerances design, an engineer should consider not only the impacts of attainable in the field and agree that this is an area that needs future surface maintenance (as the authors recommended), attention. In our study, we observed a number of locations but also the tolerances to which the field crews will build a where the initial design was sound but construction or main· short vertical curve. It may be that without very close super tenance operations changed the surface of the road such that vision, the field crews will actually build short tangents instead there was a potential for hangups. of the intended vertical curves. We concur with the discussant's remarks about use of the In the same section, the authors note that ground clearances 85th-percentile value. We proposed this as a starting point as low as 2 in. have been found; because such values have for discussions because it did have some precedence in traffic "precedence in traffic engineering," they favor 85th-percentile engineering practice. Certainly, the value or values chosen values for ground clearance and wheelbase design. I believe here should reflect a balance between the economic realities that the use of the 85th-percentile value in establishing speed faced by highway agencies and the needs of the trucking in limits or other traffic engineering criteria to which the authors dustry. Open discussion of this issue would be desirable, in were alluding does not necessarily justify its use for underside our view. clearance design. An alternative approach for arriving at the The suggestion about developing several low-clearance de proper percentile for design would involve considering the sign vehicles has considerable merit. However, there are not attributes of the particular issue at hand, in this case the sufficient data to accomplish this now. As noted in our rec· Eck and Kang 89 ommendations, we believe that it is important that additional clearance vehicles become a formal part of any design policy. data on the physical characteristics of low-clearance vehicles We hope that discu sion relative to this topic will continue be collected from all regions of the country. Such information to take place. Interested readers are encouraged to respond would be useful in establishing the vehicle dimensional re formally or informally to R. W. Eck. quirements proposed by the discussant. As the discussant clearly points out, there is still consid Publication of this paper sponsored by Committee on Geometric erable work to be done before criteria to accommodate low- Design.