Published in 2018
The Virginia Department of Transportation (VDOT) Road Design Manual requires that new commercial entrances meet certain minimum spacing standards depending on a facility’s speed limit and functional classification. Landowners, however, may request an exception from these standards, leading to two questions relevant for Virginia’s access management program: how often are such requests occurring, and are such requests associated with an increase in crash risk?
Based on data available for the period 2011-2016, such exception requests occurred for approximately 15% of all commercial entrances in Virginia, a lower figure than most, but not all, other states for which this information is available. Virginia’s exception requests are diverse, covering functional classes ranging from local roads to principal arterials and speed limits from 25 to 55 mph; examination of a smaller subset of these requests showed that about one fifth included a mitigation measure, such as entrance consolidation, designed to reduce crash risk.
Examination of crash rates (e.g., crashes per year where only full years of data are used to minimize seasonal disparities) for 64 exception sites in four of the nine VDOT districts—Fredericksburg, Hampton Roads, Northern Virginia, and Staunton—showed no significant difference between the period before the entrance was constructed and the period after the entrance was constructed (p = 0.63). Using a subset of these exception sites—just 23 in the Northern Virginia District where a similar comparison site could be identified—a matched-pairs analysis was performed, in which one subtracts the change in crash rates at each exception site (from the before to the after period) from the corresponding change at each comparison site (for the same time interval). No significant difference in these changes was detected (p = 0.94).
Although these data suggest that the exception program is not associated with an increase in crash risk, the study suggests that the exception requests continue to be monitored to ensure that exception rates remain stable. To make this monitoring feasible, the study recommends that VDOT use a customized module, recently developed by staff in VDOT’s Office of Land Use, which will facilitate the tracking of exception requests.
Ryley S. Stevens, Matthew D. Dean, Catherine H. Gayner
Last updated: November 11, 2023