RAMP/MAINLINE SPEED RELATIONSHIPS AND DESIGN CONSIDERATIONS

Great care must be exercised in selecting the design speed of ramps, particularly off-ramps from high-speed freeways, to ensure that drivers slow down from the mainline freeway speed to the design speed of the ramp. The AASHTO Green Book provides guidance on the selection of ramp design speeds that are appropriate for specified mainline highway design speeds. The process of selecting an appropriate design speed for a ramp, the operational and safety problems that may arise from selecting a ramp design speed that is too low, and the geometric design and traffic control techniques that may be used to alleviate such problems are addressed. Where problems are anticipated, geometric design changes may be appropriate to increase the ramp design speed or the deceleration distance available to drivers. Traffic control devices, including advisory speed signing, may be used to communicate to drivers the need to reduce speed. Further evaluation of the effectiveness of speed-control measures is needed.