Truck Safety Considerations for Geometric Design and

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The physical dimensions that most affect the minimum turning paths of design vehicles. are the minimum turning radius the wheelbase and the inner path of the rear tire Trucks. are wider and have greater turning radii than do buses and passenger cars Therefore the. geometric design requirements for trucks are more severe than for other design vehicles. especially at intersections and when considering horizontal alignment. Elefteriadou et al 3 evaluated truck combinations that have greater offtracking and. swept path widths than single 48 foot trailers Rocky Mountain doubles B train doubles. turnpike doubles triples and larger tractor semitrailers were considered The evaluation. considered four specific roadway geometric elements that may not adequately. accommodate large trucks 3,Horizontal curves on mainline roadways. Curb return radii for right turns at surface intersections. Curb return radii and ramp terminal right turn radii on arterial. crossroads and,Horizontal curves on freeway on and off ramps. The findings suggest that there may be a substantial cost to accommodate some of these. potential truck configurations on the existing roadway network For instance to upgrade. the entire U S freeway and nonfreeway highway network to accommodate the baseline. truck 48 ft trailer it would cost an estimated 653 million 3 This reconstruction. would include upgrades to horizontal curves on mainline roadways to accommodate. offtracking curb return radii and right turn roadways for at grade intersections curb. return radii and right turn radii for ramp terminals on the arterial crossroad at freeway. arterial interchanges and horizontal curves on freeway on and off ramps. Acceleration and Deceleration Characteristics, Trucks exhibit very different operating characteristics as those displayed by passenger. cars when accelerating from a stopped position and on both upgrades and downgrades. Both the weight horsepower ratio and the steepness and length of vertical grades greatly. influence acceleration capabilities For high speed acceleration on level highways trucks. may display similar acceleration characteristics to those of passenger cars Trucks can. display a five percent increase in speed on downgrades and a seven percent or more. decrease in speed on upgrades when compared to operations on level terrain 1. From a stopped position trucks exhibit significantly different acceleration characteristics. than those of passenger cars Figure 2 shows design time and distance relationships for. trucks from a stopped position as per the Greenbook 1 Long 4 found that observed. accelerations for WB 50 trucks are 40 to 75 percent slower than those shown in Figure 2. Figure 2 Time Distance Curves for Acceleration from Stopped Position 1. Truck deceleration rates are dependent upon the tire pavement friction pavement. properties braking efficiency and tire properties AASHTO policy 1 explicitly. considers deceleration and braking distances for use in design based on passenger cars. but suggests that trucks may display additional braking lengths because of their larger. size and vehicle weight Fambro et al 5 reported that antilock braking systems. provided improved stopping capability and that loaded stopping distances were. significantly shorter than the empty truck stopping distances Harwood et al 6. developed truck deceleration rates for empty tractor trailers on a wet pavement for the. worst and best performance drivers as well as for an antilock braking system The. results of these tests are presented in Table 1, Table 1 Truck Deceleration Rates for Use in Highway Design 6. Deceleration Rate g,Vehicle Speed AASHTO Policy Worst.
mph passenger cars Best Performance Antilock Braking. Performance,Driver System,20 0 40 0 17 0 28 0 36,30 0 35 0 16 0 26 0 34. 40 0 32 0 16 0 25 0 31,50 0 30 0 16 0 25 0 31,60 0 29 0 16 0 26 0 32. 70 0 28 0 16 0 26 0 32,1 mph 1 61 km h, The values in Table 1 are based on an assumed driver control efficiency of 0 62. conservative worst case scenario and a driver control efficiency of 1 00 best case. performance scenario The antilock braking system is shown to have very similar. deceleration rates as that for passenger cars,Swept Path Widths. Larger turning vehicles exhibit offtracking characteristics Offtracking is a function of a. truck s spacing between tire axles The maximum distance between a truck s front lead. axle and its rear trailer axle determines offtracking It is measured from the center of. the rear trailer axle with respect to the center of the lead axle Offtracking can occur in. low speed operating environments intersections or in high speed operating. environments highway horizontal curves Of interest in the low speed environment are. intersection curb return radii Horizontal curve widening is of interest in the high speed. operating environment, Table 2 shows typical minimum turning radii of various truck design vehicles as noted in.
the Greenbook The table shows that the semi trailer full trailer combination has the. largest minimum inside turning radius 22 2 feet as measured by the inside wheel path. The turnpike double semi trailer has the largest minimum design turning radius 60 feet. as measured from the outside wheel path, Table 2 Minimum Turning Radii of Truck Design Vehicles 1. Design Semi Semi Trailer Semi Trailer Interstate Interstate Triple. Vehicle Trailer Combination Full Trailer Semi Semi Semi. Type Intermediate Large Combination Trailer Trailer Trailer. Symbol WB 40 WB 50 WB 60 WB 62 WB 67 WB 96 WB 114,Turning 40 45 45 45 45 50 60. 18 9 19 2 22 2 9 1 00 20 7 17,1 m 3 28 ft, Elefteriadou et al 3 examined the impacts of current and proposed truck configurations. on the geometric and traffic operational elements on the current U S roadway network. Based on 90 degree right turn maneuvers at intersections it was found that a 45 foot. semi trailer and western twin trucks can successfully negotiate the turn at intersections. with a 30 foot curb return radii all other trucks would encroach on the opposing travel. lanes 3 When expanding the curb return radii to 60 feet only the 57 5 foot semi trailer. trucks Rocky Mountain doubles and Turnpike doubles would encroach the opposing. travel lanes while completing the right turning maneuver Lastly for curb return radii of. 100 feet all but the Turnpike double with 53 foot trailers can negotiate the turn without. encroaching the opposing travel lanes 3, Pavement widening on curves is used in high speed areas so that trucks can negotiate. curves under conditions that are similar to tangent sections The Greenbook 1 suggests. curve widening values on two lane pavements one or two way for open highways. These values are a function of the degree of curve the pavement width and the design. speed of the roadway Table 3 presents the curve widening values for cases where the. widening is 2 0 feet or greater, Table 3 Design Values for Pavement Widening on Highway Curves 1.
Degree 24 Feet 22 Feet 20 Feet, of Design Speed mph Design Speed mph Design Speed mph. Curve 30 40 50 60 70 30 40 50 60 70 30 40 50 60,2 2 0 2 0 2 0 2 5. 3 2 0 2 0 2 0 2 5 2 5,4 2 0 2 0 2 0 2 5 2 5 3 0,5 2 0 2 0 2 5 2 5 2 5 3 0. 6 2 0 2 0 2 5 2 5 3 0 3 0 3 5,7 2 0 2 5 2 5 3 0 3 5. 8 2 0 2 0 2 5 3 0 3 0 3 5,9 2 0 2 0 2 5 3 0 3 0 3 5 4 0.
10 11 2 0 2 5 3 0 3 5,12 14 5 2 0 2 5 3 0 3 5 4 0,15 18 2 0 3 0 4 0. 19 21 2 5 3 5 4 5,22 25 3 0 4 0 5 0,26 26 5 3 5 4 5 5 5. 1 mph 1 61 km h, Elefteriadou et al 3 found that all combination trucks traveling at the roadway design. speed would not encroach on adjacent lanes or shoulders of roadways or ramps designed. in accordance with the Greenbook s high speed design criteria On the other hand if. trucks were traveling at very low speeds on the sharpest horizontal curves 30 mph design. speed with 73 ft radius suggested by the Greenbook only the turnpike double trucks. would require curve widening on ramps from 15 to 16 feet. SIGHT DISTANCE, The following section describes sight distance considerations for trucks Included is. recent literature pertaining to stopping sight distance SSD intersection sight distance. ISD and passing sight distance PSD,Stopping Sight Distance.
SSD is determined by the reaction time and braking distance required for an alert driver. traveling at or near the design speed to react and stop before hitting a stationary object. on a wet roadway 1 The recommended minimum SSD calculated according to the. Greenbook procedures are based on passenger car operation yet large trucks require. longer braking distances than passenger cars The current policy is based on the distance. traveled during perception and reaction and the distance traveled during braking The. current Greenbook SSD equation takes the following form 1. where V design speed mph,t perception reaction time assumed 2 5 sec. f coefficient of friction between tires and roadway and. G grade decimal, The above equation assumes a driver eye height of 3 5 feet and an object height of 6. Research has shown that most large trucks are capable of stopping within AASHTO. design braking distances on dry pavements With anti lock braking systems they are. also capable of stopping within AASHTO design braking distances on wet pavements. The additional stopping distance is balanced by the fact that the truck operator is able to. see vertical features of obstruction farther ahead than passenger cars because of the. higher position of the seat in the vehicle When horizontal sight restrictions occur on. downgrades particularly at the ends of long downgrades there is little advantage to the. greater eye height of the truck operator In this situation the SSD provided should be. more than the minimum 5, The SSD model that is being incorporated into the 2000 edition of the Greenbook is an. explicit model that takes the following form 5,where V initial speed mph. t brake reaction time assumed 2 5 sec,a driver deceleration assumed 11 2 ft sec2 and.
g gravitational constant 32 2 ft sec2, The underlying assumptions of the model are that the driver eye height is 3 5 feet and the. object height is 2 0 feet Where heavy vehicles are the design vehicle an eye height. between 7 5 feet and 8 5 feet is appropriate Table 4 compares the braking distances for. passenger cars and trucks 5 Truck braking performance is based on both the best and. worst case drivers and on anti lock brake systems For both the best and worst case. performing drivers it can be concluded from Table 4 that truck braking distances are. greater than those for passenger cars When trucks are equipped with anti lock brakes. the braking distance for trucks decreases considerably. Table 4 Truck Braking Distance on Wet Pavement 5,Braking Distances for Trucks ft. AASHTO Criteria,Design Speed Worst, for Passenger Cars Best performance Anti lock Brake. mph performance,ft Driver System,20 33 77 48 37,30 86 186 115 88. 40 167 344 213 172,50 278 538 333 267,60 414 744 462 375.
70 583 1013 628 510,Intersection Sight Distance, There are five cases for determining intersection sight distance ISD. Case I Intersection with no control,Case II Intersection with yield control. Case III Intersection with stop control,o IIIA Crossing maneuver. o IIIB Left turn onto a major highway,o IIIC Right turn onto a major highway. Case IV Intersection with signal control, Case V Stopped vehicle turning left from a major road.
The ISD for stop controlled intersections is a conservative estimate for all five types of. intersection control There are three subcases to consider when determining the ISD for a. stop controlled intersection The left and right turning subcases IIIB and IIIC require a. longer ISD and therefore generally govern design The AASHTO policy for determining. ISD only considers truck requirements explicitly in case IIIA The current policy. suggests the following equation to calculate the ISD for Case IIIA 1. where ISD sight distance to left d1 or right d2 along the major road from the. intersection required for the minor road vehicle to cross the major road. V design speed of major road mph, J sum of perception time and time required to actuate the clutch or and. automatic shift assume J 2 0 sec, ta time required to accelerate and traverse the distance to clear the major. S D W L the distance that the crossing vehicle must travel to clear. the major road ft, D distances from the near edge of pavement to the front of a stopped. vehicle ft assume D 10 ft, W intersection width along path of crossing vehicle ft and. L overall length of minor road vehicle ft, Trucks are not considered in the current AASHTO policy for the controlling subcases.
IIIB and IIIC It is recommended that criteria be developed for consideration in the. design of stop controlled intersections with a substantial number of trucks either crossing. or entering the major road Harwood et al 7 recommended travel times critical gaps. to determine sight distances for turning left or right and for crossing two lane highways. Table 5 compares left and right turn travel times that can be used to determine the leg of. the departure sight triangle along the major road for passenger cars single unit trucks. and combination vehicles Table 6 compares crossing maneuver travel times used to. determine the leg of the departure sight triangle along the ma. travel lanes while completing the right turning maneuver Lastly for curb return radii of 100 feet all but the Turnpike double with 53 foot trailers can negotiate the turn without encroaching the opposing travel lanes 3

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