Flatter than minimum curvature for any particular design speed should be used where possible, while retaining the minimum guidelines for the most critical conditions.

Alignment consistency should be sought. Sharp curves should not follow long tangents or a series of flat curves.

Sharp curves should be avoided on long, high fills. It is difficult for drivers to perceive the extent of curvature and adjust their operation accordingly when the adjacent topography does not extend above the level of the roadway.

Compound curves (two adjacent curves in the same direction with different radii) should be used with caution and should be avoided on mainlanes where conditions permit the use of simple curves. Where compound curves are used, the ratio of the flatter radius to the sharper radius should not exceed 3:2 (i.e., R

1

should not exceed 1.5R

2

). For intersections or other turning roadways (such as loops, connections, and ramps), this ratio may be increased to 2:1 (i.e., R

1

may be increased to 2R

2

).

Reverse curves (two adjacent curves in opposite directions) on high-speed facilities should include a tangent section of sufficient length to provide adequate superelevation transition between the curves.

Broken-back curves (two curves in the same direction with a short tangent between the curves) should be avoided except where very unusual topographical or ROW conditions make other alternatives impractical. This configuration is unexpected by drivers, not pleasing in appearance, and more difficult for freight truck maneuverability.

It is recommended to provide a minimum tangent length that is at least 15 times the design speed.

A design exception is not required if this recommended tangent length is not met.

Horizontal alignment and its associated design speed should be consistent with other design features and topography. Combination of horizontal and vertical alignments is discussed in .