Deceleration Lengths for Exit Terminals
Publication: Journal of Transportation Engineering
Volume 138, Issue 6
Abstract
General freeway design practice assumes deceleration for a vehicle exiting the freeway occurs between the point when the vehicle clears the through-traffic lane and the point of limiting design speed for the ramp proper. The length provided between these points should be at least as great as the distance needed to accomplish 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 used in design are based on assumed running 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 had been assumed for several design elements, including exit ramps. Although the assumption that drivers choose speeds less than the 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 on the basis of research, including studies on speed behavior on highways and detailed decelerating behavior for 27 exit maneuvers at two ramps. The paper includes suggested deceleration lengths.
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Acknowledgments
This paper is based on research sponsored by the Texas Department of Transportation (TxDOT) and the U.S. Department of Transportation, Federal Highway Administration (FHWA). The study from which the driver braking behavior were drawn was sponsored by the Harris County Toll Road Authority (HCTRA). The work was performed by the Texas Transportation Institute. The contents of the paper reflect the views of the authors, who are responsible for the facts and the accuracy of the data presented herein. The contents do not necessarily reflect the official views or polices of TxDOT, FHWA, or HCTRA. The authors recognize Jesse Stanley for his assistance with accessing the vehicle data from the toll road study, Karl Zimmerman for his assistance with reviewing the identified equations and variable values used to calculate ramp deceleration lengths in the Green Book, and Maria Burke who served as the project director for the TxDOT study that examined the history for several controlling criteria.
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© 2012. American Society of Civil Engineers.
History
Received: Feb 4, 2010
Accepted: Dec 1, 2011
Published online: May 15, 2012
Published in print: Jun 1, 2012
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