E 410 Horizontal Alignment

The general considerations for determining horizontal alignment are not necessarily the same for rural or outlying areas as for urban areas.  Therefore, the discussion of horizontal alignment that follows will include the general design policy as well as the use of superelevation on horizontally curved alignment and its limiting and modifying effect on City Streets.

Alignment should be as direct as possible consistent with topography. A balance should be struck between the two extreme alternatives of constructing a straight line between two points and following every curve or contour in the existing topography. Sudden sharp curves should not be introduced at the end of long tangents or long radius curves.  When physical conditions necessitate the use of a lower than standard radius for a given design speed, the substandard radius should be gradually approached by successively sharper curves from the tangent or long radius curved sections. Horizontal alignment should provide at least a safe stopping sight distance, for a given design speed, throughout the entire length of the project. The criteria for sight distance are discussed further in this chapter.

Long radius curves should be used wherever economically feasible, reserving the minimum radius of curvature for use at only the most critical sections, such as where heavy cuts or fills or extensive right-of-way costs may be involved. A length of arc as long as possible should be used for curves with small deflection angles in order to prevent the appearance of a kink in the road alignment. See Section E 411.6, Minimum Length of Curve for minimum length of curve that may be used.

Compound curves (other than certain curb return radii on channelization projects) should be used only where the topography, construction right of way costs, or other conditions make it impractical to use a single curve.  Where the use of a compound curve is unavoidable, and the shorter radius is less than 1,000 feet the shorter radius should be at least two-thirds the length of the longer radius.

Reverse curves without the use of an adequate length of intervening tangent are undesirable because they produce a relatively abrupt change of alignment, making it more difficult for a driver to follow and stay in the same lane. The proper amount of superelevation may be difficult or impossible to apply throughout the entire length of the reversing system because of insufficient length of tangent to accommodate the superelevation runoff. Superelevation runoff is the general term denoting the change in cross-section from a normal crown section to the fully superelevated section, or vice versa.

A broken-back curve consists of two curves in the same direction connected by a short tangent. This type of alignment is not pleasing in appearance and is difficult to negotiate because it is not generally anticipated by the driver.  Unless the use of broken-back curves is unavoidable, the alternate methods of alignment, listed in the order of desirability, that should be used are:

1. Increasing the length of intervening tangent.
2. Providing one single curve.
3. Using a compound curve.

The broken-back curve usually makes it necessary to carry superelevation across the intervening tangent portion. This may or may not be desirable. The first alternative may eliminate or reduce the superelevation of the tangent portion. The last two alternatives enable some degree of continuous superelevation to be maintained. See Section E 411, Superelevation. Where feasible, a curve beginning or ending near a bridge or grade separation should be so located that the superelevation transition does not take place on the bridge or the main structure of the grade separation.