FLH PDDM Chapter 9 - Eastern Federal Lands Highway Division

9.4.A. Geometric Design. (continued)At the beginning of a design, the depth of the pavement structure may be arrived at by an assumptionbased on experience or by comparing with the depths used on an adjacent project. Following ageotechnical investigation, the designer will adjust the assumed depth accordingly. The geotechnicalinvestigation usually takes place after a line and grade have been established.For RRR projects, the riding quality of an asphalt surface may be improved by providing a leveling course.This additional depth may increase the pavement structure capabilities and merits consideration in the finalpavement design when leveling is relatively uniform over the length of the project. If a field review of theproject is not practical, the designer should increase asphalt concrete pavement quantities by 20 percent foruse as leveling material. See Chapter 6 for additional details on the design of asphalt and concretepavements.b. Profile Grade Location and Cross Slope. The standard location of profile grade on the highway crosssection is at centerline or low side of the superelevated section for all two-lane highways.The cross slope on tangents on paved highways shall be from 1.5 to 2.0 percent.Normally, the cross slopes on gravel surfaced roads shall be 3 to 4 percent.The shoulder cross slope should be the same as the adjacent traffic lane. With curb sections or when theshoulder surface is an asphalt surface treatment, aggregate, or turf, increasing the slope helps to facilitatedrainage. In these cases, consider cross slopes of 4 to 6 percent. On super elevated curves, the roll-over incross slope on the outside of the curve should not exceed 8 percent.The cross slope on the tops of base courses and the subgrade is usually the same as on the finishedpavement. In some cases it is desirable to have a reverse slope on the subgrade (on the high side of curvesand outside the edge of the pavement) to prevent moisture from entering the base.c. Lane and Shoulder Widths. The Green Book and other agency standards show lane and shoulderwidths for each functional classification for various design speeds and traffic volume ranges.When the percentage of trucks or recreational vehicles is high in comparison to the ADT, considerincreasing lane widths.d. Foreslopes. Foreslopes ensure the stability of the roadway and provide a reasonable opportunity forrecovery of an out-of-control vehicle. The foreslope, the slope from the edge of the surfaced shoulder tothe edge of the subgrade shoulder, should not be steeper than the slope ratios shown in Table 9-7, unlessguardrail is placed (Detail designs should be in accordance with the Roadside Design Guide, AASHTO,1996).The slope from the edge of the subgrade shoulder to the bottom of the ditch should normally be anextension of the foreslope.9 - 40