Selecting Optimal Deceleration-Lane Lengths at Freeway Diverge Areas Combining Safety and Operational Effects

Document Type

Conference Proceeding

Publication Date



Until now, the findings of impacts of deceleration lane lengths on safety were not quite consistent or were even contradictory. A comprehensive study was needed to have a better understanding of the effects of different deceleration lane lengths on traffic safety and operations. This study has three objectives: 1) evaluate the safety performance of different deceleration lane lengths at freeway diverge areas; 2) examine the operational effects of deceleration lane lengths for two design types (one-lane exits with parallel/tapered designs and two-lane exits with parallel design); and 3) select optimal deceleration lane lengths by combining the results from safety and operation aspects. A total of 218 sites, categorized into nine groups, were selected for the crash analysis. A total of 360 simulation models were developed for different scenarios by the combination of the exit types (one-lane exits/two-lane exits), design speeds, exiting volumes, and number of through-lanes. The total delays of through-traffic were measured. The safety and operational analysis results suggest that (1) for one-lane exits, a minimal deceleration length of 500 ft is essential for the design speed of 55 mph, 600 ft for the design speeds of 60 and 65 mph, and 700 ft for the design speed of 70 mph; (2) for two-lane exits, the minimal deceleration length of 500 ft is suggested for the design speeds of 55 mph and 60 mph, 600 ft for the design speeds of 65 and 70 mph; and (3) for both one-lane and two-lane exits, deceleration lane lengths longer than 700 ft are not recommended from a safety perspective. The results of this study could be used as a supplementary to the current design guideline.

Was this content written or created while at USF?


Citation / Publisher Attribution

TRB 91st Annual Meeting, Compendium of Papers, Transportation Research Board, Washington, D.C., January 22–26, 2012.