Establishing Advisory Speeds on Freeway Exit Ramps

Guidance on exit ramp advisory speeds is limited and based primarily on horizontal curvature. The generality of the guidance provides an engineer with flexibility, but likely also results in inconsistent exit ramp advisory speed signing practices. In addition, non-direct connectors (i.e., regular slip ramps) where there are no obvious speed-restricting alignment features, but where a speed transition is needed or desired, is not clearly addressed. The research documented in this paper includes a survey of state department of transportation (DOT) exit ramp advisory speed signing practice as well as speed studies on exit ramps in Texas. A range of horizontal and vertical curvatures, freeway-to-frontage-road speed differentials and distances between ramp locations and the nearest downstream cross street intersection were represented in the data. Speed profiles were developed for each location and qualitatively examined to assess cases with good and poor agreement between ramp advisory speeds and operating speeds and to also identify speed-influencing ramp features. A predictive model of exit ramp speeds was also estimated using the degree of horizontal curvature and the distance to the nearest downstream intersection as predictors. Model predictions for mean truck speeds were used as inputs to a recommended procedure for determining exit ramp advisory speeds.