Review of Truck Characteristics as Factors in Roadway Design (2003) / Chapter Skim
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From page 61... ...
61 CHAPTER 6 HIGHWAY GEOMETRIC DESIGN CRITERIA AND THEIR RELATIONSHIP TO TRUCK CHARACTERISTICS This chapter provides a review of the appropriateness of individual highway geometric design criteria to accommodate trucks. The review includes the following highway geometric design criteria: • Stopping sight distance, • Passing sight distance and passing/no-passing zones on two-lane highways, • Decision sight distance, • Intersection sight distance, • Railroad-highway grade-crossing sight distance, • Intersection and channelization geometrics, • Critical length of grade, • Downgrades, • Acceleration lanes, • Deceleration lanes, • Lane width, • Horizontal curve radius and superelevation, • Pavement widening on horizontal curves, • Cross-slope breaks, and • Vertical clearance.
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From page 62... ...
The first term in Equation 25 represents the brake reaction distance and the second term represents the braking distance. Equation 25 is not conceptually different from the stopping sight distance models used in previous editions of the Green Book, but the parameters of the model are now defined in ways that more realistically represent traffic situations encountered in emergency maneuvers.
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From page 63... ...
Application of Stopping Sight Distance Criteria to Horizontal Curves Sight distance can also be limited by obstructions on the inside of horizontal curves, such as trees, buildings, retaining walls, and embankments. Horizontal curves designed in accordance with the Green Book should provide sight distance at least equal to the design values in Table 39 along the entire length of the curve.
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From page 64... ...
as appropriate for most drivers in several older studies (28, 58)
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From page 65... ...
Driver Eye Height. The minimum crest vertical curve criteria for stopping sight distance in Table 40 are based on a driver eye height for passenger cars of 1,080 mm [3.5 ft]
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From page 66... ...
• The 1984 Green Book stopping sight distance criteria were adequate for trucks with conventional brake systems and the best-performance driver at vertical sight restrictions and were nearly adequate at horizontal sight restrictions. • The 1984 Green Book stopping sight distance criteria were not adequate to accommodate trucks with conventional brake systems and poor-performance drivers, but changes in stopping sight distance criteria to accommodate poor-performance drivers would only be cost-effective for new construction or major reconstruction projects on rural two-lane highways that carry more than 800 trucks per day and rural freeways that carry more than 4,000 trucks per day.
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From page 67... ...
of the left lane is 16 km/h [10 mph] higher than that of the passed vehicle.
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From page 68... ...
The Green Book policy estimates the time for the initial maneuver (t l) as within the 3.6 to 4.5 s range, based on field data.
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From page 69... ...
Table 41 illustrates the derivation of the Green Book passing sight distance criteria, representing the sum of the distances dl through d4 for specific speed ranges. Table 42 presents the Green Book passing sight distance criteria for specific design speeds.
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From page 70... ...
Minimum Passing Zone Length Another consideration in the marking of passing and nopassing zones on two-lane highways is the minimum length of a passing zone. The Green Book does not address passing zone lengths at all.
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From page 71... ...
where ∆C = critical separation (distance from front of passing vehicle to front of passed vehicle at critical position)
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From page 72... ...
that could be used for the minimum length of a passing zone, including the implicit MUTCD criteria, the sum of distances dl and d2 based on the assumptions in Green Book policy, and the 85th percentile value of the sum of distances dl and d2 based on field observations (2,3)
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From page 73... ...
A truck is also not likely to use a deceleration rate as high as 2.4 m/s2 [0.25 g or 8 ft/s2] in aborting a pass except in an emergency situation.
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From page 74... ...
The figure indicates that the MUTCD criteria are in good agreement with the requirements for a passenger car passing another passenger car. The other passing scenarios -- passenger car passing truck, truck passing passenger car, and truck passing truck -- each require progressively more sight distance, but all are substantially less than the current AASHTO Green Book criteria.
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From page 75... ...
passing vehicle is assumed to accelerate at a constant rate, a, until the desired speed differential, m, relative to the passed vehicle is reached. Thus, tl can be calculated as m/a.
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From page 76... ...
76 cars in those zones. Clearly, this would reduce the level of service on two-lane highways.
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From page 77... ...
and complete the selected maneuver safely and efficiently (1)
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From page 78... ...
78 be cost-effective. A similar analysis indicates that changes to the decision sight distance criteria in the 2001 Green Book to better accommodate trucks would still not be cost-effective.
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From page 79... ...
that exceed stopping sight distances are desirable along the major road. Prior to the 2001 Green Book, intersection sight distance policies were presented based on a kinematic or accelerationdeceleration model.
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From page 80... ...
low-volume intersections at which the appropriate design vehicle is unlikely to be a truck. Case B -- Intersections With Stop Control on the Minor Road Departure sight triangles for intersections with stop control on the minor road are considered for three situations: • Case B1 -- Left turns from the minor road; • Case B2 -- Right turns from the minor road; and • Case B3 -- Crossing the major road from a minor-road approach.
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From page 81... ...
angle along the major road in both directions is the recommended intersection sight distance for Case B1. The vertex (decision point)
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From page 82... ...
provide sufficient sight distance for a passenger car to cross the near roadway to reach the median. Possible exceptions are addressed in the discussion of Case B3.
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From page 83... ...
onto the major road. The intersection sight distance for right turns is determined in the same manner as for Case B1, except that the time gaps (tg)
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From page 84... ...
the major road to accommodate the crossing maneuver (distance b in Figure 41a) should be computed with Equation 39.
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From page 85... ...
intersection, but is not needed at a stop-controlled intersection (Case B)
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From page 86... ...
stopped on each of the other approaches. Left-turning vehicles should have sufficient sight distance to select gaps in oncoming traffic and complete left turns.
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From page 87... ...
vehicle, because a vehicle that turns left without stopping would need less sight distance. The Green Book criteria for sight distance along the major road to accommodate left turns is the distance traversed at the design speed of the major road in the travel time for the design vehicle as shown in Table 57.
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From page 88... ...
criteria for these cases are presented in the Green Book, but have been adapted from two other publications (82, 83)
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From page 89... ...
The Green Book states that corrections should be made for skewed crossings and for highway grades other than flat. The Green Book states that sight distances of the order shown in Table 58 are desirable at any railroad grade crossing not controlled by active warning devices, but that their attainment is difficult and often impractical, except in flat, open terrain.
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From page 90... ...
• Turning roadways with corner islands (Chapter 9, p.
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From page 91... ...
is defined as the path of the outer front wheel, following a circular arc, at a speed of less than 16 km/h (10 mph) , and is limited by the vehicle steering mechanism.
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92 Figure 46. Case B: departure of vehicle from stopped position to cross single railroad track (1)
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From page 93... ...
left-turn lane, the Green Book does not go into further detail on the design of double left-turn lanes. The primary factor to consider in designing double leftturn lanes is vehicle offtracking or swept path width.
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From page 94... ...
through the roundabout can result in loss-of-load incidents for trucks; however, it is indicated in the Roundabout Guide that drivers generally expect to travel at slower speeds and will accept the higher side force caused by a reasonable superelevation rate. In summary, it is recommended that the Green Book section on roundabouts be expanded to incorporate the design guidelines developed in the Roundabout Guide, particularly those shown in Tables 60 and 61.
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From page 95... ...
95 Figure 47. Speed-distance curves for a typical heavy truck of 120 kg/kW [200 lb/hp]
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based on judgment, rather than actual field data and, therefore, merits closer review. Specific comments on the AASHTO criteria are presented below.
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From page 97... ...
97 Figure 48. Critical lengths of grade for design, assumed typical heavy truck of 120 kg/kW [200 lb/hp]
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From page 98... ...
The major dissipater of excess energy in trucks is normally its brakes. The energy absorbed by the brakes is converted into heat, raising the temperature of the brake linings, brake drums/disks, and wheel assemblies.
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analysis of truck performance on downgrades that was recommended for incorporation in FHWA's Interactive Highway Safety Design Model (IHSDM)
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100 TABLE 62 Minimum acceleration lengths for entrance terminals with flat grades of 2 percent or less (1) TABLE 63 Speed change lane adjustment factors as a function of grade (1)
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lanes should be provided on elevated freeways where entrance ramps must necessarily incorporate upgrades. Critique of Geometric Design Criteria An evaluation of Table 62 was conducted using the truck speed profile model (TSPM)
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From page 102... ...
the range of 65 to 110 lb/hp have sufficient acceleration capabilities to achieve the given speeds within the minimum acceleration lengths. Considering that the 2001 Green Book indicates a 200-lb/hp truck is representative of the size and type of vehicle normally used for design control of major highways and that current field data indicate that on the freeways the 85th percentile weight-to-power ratios of trucks falls within a fairly narrow range around 170 to 210 lb/hp (see Appendix D)
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no truck traffic. However, both 3.3- and 3.6-m [11- and 12-ft]
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vehicle operations that vehicles not rollover on horizontal curves. The tendency of the vehicle to overturn must be resisted by the roll stability of the vehicle.
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From page 105... ...
site occurs as a vehicle leaves a horizontal curve. In fact, there is a gradual rather than an instantaneous change in lateral acceleration, because drivers steer a spiral or transition path as they enter or leave a horizontal curve.
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From page 106... ...
some point on each curve, the friction demand of each vehicle will be slightly higher than suggested by Equation 44. Oversteering by passenger cars is not considered in the AASHTO design policy for horizontal curves, but it is probably not critical because the AASHTO maximum lateral acceleration requirements are based on driver comfort levels rather than the available pavement friction.
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From page 107... ...
Margin of Safety Against Skidding Current design criteria for horizontal curves are intended to maintain the vehicle lateral acceleration within driver comfort levels that are below the lateral acceleration at which the vehicle would skid on a wet pavement. The vehicle's lateral acceleration is resisted by superelevation and tire-pavement friction.
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108 TABLE 68 Margins of safety against skidding on horizontal curves Passenger car Truck Design Speed (mph) Maximum superelevation e Maximum tolerable lateral acceleration (g)
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The margins of safety for trucks in Table 67 are large enough to provide safe truck operations if there are no major deviations from the basic assumptions used in horizontal curve design. The effects of deviations from the basic assumptions are considered below.
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110 TABLE 69 Margins of safety against rollover on horizontal curves Passenger car Truck Rollover margin of safety Design Speed (mph) Maximum e Maximum tolerable lateral acceleration Minimum radius (ft)
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The Green Book criteria for tire-pavement friction are based on a poor, wet pavement and (apparently) on worn tires.
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From page 112... ...
112 TABLE 70 Vehicle speed at impending skidding or rollover on horizontal curves Passenger car speed (mph) Truck speed (mph)
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and tolerable to the driver. A 1971 Dodge Coronet was the passenger car used in the simulations.
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