Operational Evaluation of Freeway Ramp Design

Abstract

Ramp facilities provide all freeway entrance and exit opportunities. Thus, excellent freeway ramp design criteria are critical to freeway operations and safety. Current freeway entry ramp design speed criteria were evaluated by observing six (nonloop) ramps in three Texas cities. Ramp and freeway traffic speed-distance relationships were observed in the field using videotaping methods. Traffic operations were described in terms of ramp and freeway speeds and accelerations, as well as merging locations, accepted time gap sizes, and freeway time headways. Performance characteristics for numerous ramp features and volume combinations were compared. Combined design feature and traffic volume effects on freeway operations and both positive and negative impacts that may be attributed to ramp design features were evaluated. Several notable conclusions were reached. First, for virtually all observations, ramp driver speeds were found to be greater than 50 percent of the freeway design speed; therefore, a 50th percentile design speed might have negative safety implications. Second, the ability of entry ramp drivers to see freeway right-lane traffic, into which merging is intended, before reaching the ramp gore was found to be very important. Therefore, the AASHTO acceleration lane length measurement model for taper-type ramps should be clarified. The acceleration lane should be considered to begin only when ramp drivers have an unobstructed view of freeway right-lane traffic.