Chapter 4 - Elements of Design

4.5. Superelevation

4.5.4. Superelevation Transitions (continued)

Minimum Length of Superelevation Runoff

There are a number of rational approaches to transitioning from a normal crown section to a superelevated section. Wherever possible, GDOT applies 2/3 of the superelevation runoff outside the curve and 1/3 of the superelevation runoff inside the curve. For the above example, the amount of superelevation applied outside the curve would be (2/3)(233.33) = 155.56-ft. and the amount of superelevation applied inside the curve would be (1/3)(233.33) = 77.78-ft.

Tangent runout is the length required to transition from a normal crown to a flat section on the outside of a horizontal curve. The tangent runout length is determined in the same manner as the superelevation runoff length. For the example above, assuming a normal crown cross slope of 2%), the tangent runout length would be:

 

length tangent runout

Calculated lengths may be rounded to the nearest foot, if desired. 

If geometric constraints exist, consideration may be given to placing 50% of the superelevation runoff on the tangent and 50% of the runoff on the curve. Sometimes, conditions exist where it is not possible to develop the desirable amount of runoff (or runout) and it is impossible to locate the transition in the ideal location relative to the curve PC or PT.

Examples of this include:

• Reverse curves (especially prevalent in mountainous regions)
• Broken back curves
• Approaches to intersections

These undesirable situations should be avoided, wherever feasible. However, since these instances are sometimes unavoidable (or the desirable implementation is impractical) – professional judgment should be exercised when determining less-than-ideal transition rates and transition locations. Some practical guidelines for handling these situations include:  

• For a symmetric (equal radius) reverse curve, place the 0% cross slope point at the PT and PC common to both curves
• For asymmetric reverse curves (of different radii), attempt to place the superelevation transition in a location which is proportional to the emax of the two curves
• For broken back curves, attempt to place the average emax cross slope at the center of the tangent
• Pavement warping near intersection tie-ins is sometimes required (e.g. when there are superelevation transitions near intersections, ADA requirements, drainage, sight distance, and operations which should be taken into consideration)

Figure 4.1., Crowned Traveled Way Revolved About Centerline, illustrates the development of tangent runout and superelevation runoff for a roadway with the profile control and superelevation rotation point at the center of the roadway.

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 4.1. Crowned Traveled Way Revolved About Centerline