FLH PDDM Chapter 9 - Eastern Federal Lands Highway Division

9.4.A. Geometric Design. (continued)The designer should plot a horizontal alignment through the established control points using splines, curvetemplates, shop curves, or freehand methods. These preliminary layout stages avoid or limit the use ofstraightedges and string lines to evaluate a curvilinear design properly.In rolling or mountainous terrain, it is desirable to plot a rough profile to determine if the preliminaryalignment will fit the vertical controls. This consists of scaling stations on the horizontal alignment andpicking elevations from the contours.A rough profile plot on a reduced scale ratio (such as 1:5000 horizontal and 1:500 vertical) is adequate todetermine the need for alignment shifts. Several adjustments of the rough line and grade may be necessarybefore a reasonably good initial line complies with the design speed requirements.4. Horizontal Alignment. Horizontal alignment is a combination of circular curves, transition curves, andtangents. Horizontal alignment must provide for safe and continuous operation at a uniform design speedfor substantial lengths of highway.The major design considerations in horizontal alignment are safety, functional classification, design speed,topography, vertical alignment, construction cost, cultural development, and aesthetics. These factors,when properly balanced, produce an alignment that is safe, economical, and in harmony with the naturalcontour of the land.The following guidelines apply to all alignment projections:The line should be as directional as possible, consistent with topography and land use. A flowing linefollowing the natural contours is preferable to one with long tangents slashing through the terrain andcausing large construction scars.If possible, avoid the use of the minimum radius for the design speed.Consistent alignment is the desirable end product. Sharp curves introduced at the ends of longtangents, and sudden changes from flat curvature to sharp curvature are dangerous. When sharpcurvature is used, successively sharper transition curves from flat curvature to sharp curvature areapplicable. This is necessary since actual operating speeds typically exceed design speeds on long flatcurves (radius>450 m) and tangents. The designer may assume 85th percentile operating speeds of100 km/h approaching curves following tangents or flat curves longer than 500 meters.On long high through-fills, use only very flat curvature unless guardrail or other measures such asplantings or reflectors are used to delineate the edge of the roadway.Small deflection angles should have long curves to avoid the appearance of a kink. On alignmentswith long tangents and sight distances, the curve should be at least 150 meters in length for a deltaOO(central angle) of 5 . This minimum length increases 30 meters for each 1 decrease in delta. Forcurves located on a crest vertical curve, decreasing the above lengths will still provide a pleasingtransition.Deflections of 15 minutes and less do not require the use of a curve, but it is preferable to locateslight breaks in grade at these angle points to minimize the visible effect to the road user.For aesthetic reasons and driving comfort, the preferable minimum length of curve should bebetween 1.5 to 3 times the design speed.9 - 15