The AASHTOs A Policy on Geometric Design of Highways and Streets (commonly known as the Green Book) notes that vehicles exiting from a freeway should decelerate after clearing the through-traffic lane and before reachingthe point limiting design speed for the ramp proper. The length provided between these points should be at least as great as the distance needed toaccomplish the appropriate deceleration, which is governed by the speed of traffic on the through lane and the speed to be attained on the ramp. The deceleration length values in the 2004 Green Book are based upon assumedrunning speed for the limited-access highway and the ramp along with deceleration rates based on 1930s studies. The need to update the speed assumption for the highway and the ramp curve is clear although determining appropriate deceleration rates is not as simple. Previous research has demonstrated that drivers select speeds at or above the design speed on horizontal curves, rather than the much lower average running speed that previously been assumed for several design elements including exit ramps. While the assumption that drivers choose speeds less than design speed in a free-flow situation is certainly questionable, there is some evidence that drivers do decelerate in the travel lane before moving into the deceleration lane. Updates for the current assumptions were identified based on researchincluding studies on speed behavior on highways and detailed deceleratingbehavior for 27 exit maneuvers at two ramps. The paper includes suggested deceleration lengths.
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