FLH PDDM Chapter 9 - Eastern Federal Lands Highway Division

9.4.A. Geometric Design. (continued)When at-grade intersections occur on roadways with moderate to steep grades, it is desirable to reducethe grade through the intersection.In swampy terrain and areas subject to overflow and irrigation, the low point of the subgrade should beat least 0.5 meters above the expected high water. For roads located along main streams and rivers, referto Chapter 7 for the appropriate hydraulic controls.b. Maximum Grade. The designer should know the functional classification of the project from theplanning and programming process (see Chapter 2). Consider this data, pages 227-241 of the Green Book,and the type of topography to determine the maximum allowable grades in relation to design speed.Example:- Rural Area.- Highway Functional Classification is Rural Collector.- 60 km/h Design Speed.- Rolling Terrain.According to the Green Book (Table VI-3, maximum grades, page 463), the maximum grade for thisexample is 8 percent.When analyzing maximum grades, note that superelevation transitions will increase the effective grade onthe edge of the traveled way. This increase is significant particularly to trucks and recreational vehicles.To minimize this effect on long continuous runs of near maximum grades, the designer has two options:(1) Flatten the grade throughout the curve.(2) Carry profile grade on the right edge of the traveled way going upgrade.This is especially important when the design contains climbing lanes or scenic pulloffs.c. Minimum Grade. Flat and level grades on uncurbed pavements are not objectionable when thepavement is adequately crowned to drain the surface laterally.A flat grade (0.00 percent) is acceptable in through-fill sections where the highway has sufficient crown.Minimum grades (0.5 percent) are applicable only for drainage of roadway ditches in cut sections, drainageof curb sections, and to ensure pavement drainage on superelevation transitions. This requirementparticularly applies where flat grades on crest and sag verticals have substantial lengths that are essentiallyflat. It also applies where superelevation transitions introduce sags in the ditch or gutter line. Computerplots of the ditch or gutter profiles will highlight any drainage problems for correction as the designprogresses.See Section 9.4.A.7, Geometric Cross Section, for details on design of drainage.9 - 25