RAMP EXIT/ENTRANCE DESIGN--TAPER VERSUS PARALLEL AND CRITICAL DIMENSIONS

The basic design criteria, and thereby design standards, used by governmental agencies to design exit and entrance ramp terminals have not changed in more than 30 years. However, the design details of the ramp terminals vary across the country. A survey of state departments of transportation (DOTs) indicates that there is mixed use of (a) only tapered lanes, (b) only parallel lanes, and (c) a combination of both tapered- and parallel-lane design. Forty-one (91%) of 45 state DOTs that responded to a nationwide NCHRP survey prefer a tapered design for exit ramps. Thirty-four (75%) of the responding states use a parallel design for entrance ramps. Most agencies use AASHTO policies as a basis for speed-change lane design and either comply with or exceed AASHTO recommendations for deceleration lane lengths. However, some state standards indicate minimum lengths of acceleration lanes that are less than the minimum lengths recommended by AASHTO. Most research indicates that operational aspects of the current design elements are acceptable for today's driving conditions. The "gore" or "wedge" of exit ramps ranks high in the location of freeway accidents, and some problems exist with respect to driver gap acceptance on entrance ramps. Both conditions have been attributed to the assumption that drivers do not know how to properly use, or just do not properly use, speed-change lanes.