Driver Response to Variable Speed Limits on I-66 in Northern Virginia

Variable speed limits (VSLs) are components of active traffic management (ATM) systems that are increasingly being used as safety and operational treatments in urban areas. Studies have shown safety and operational benefits associated with VSLs; however, the level of benefit is dependent on the degree to which drivers comply with the VSL. Although several studies have quantified macroscopic safety and operational benefits of VSL, limited studies have examined the degree to which drivers adhere to the posted VSL signs. This study evaluated how speed changed in response to an advisory VSL system on Interstate-66 (I-66) in Northern Virginia. Compliance in this study was defined as when vehicle speeds in a time interval were at or less than the speed posted on an advisory VSL sign. The study generally revealed that noncompliance increased as the posted VSL speed increased. The analysis further showed that the observed speed of all drivers was at or below the posted VSLs’ speed 37% and 40% of the time downstream and upstream of I-66 VSL gantries, respectively. About 47% and 51% of observed interval speeds exceeded the posted VSL by more than 2.235 m/s ($5\mathrm{mi}/\mathrm{h}$) downstream and upstream of the VSL gantries, respectively. The analysis also showed relatively higher compliance levels when VSLs were spaced closer together and had lower VSL posted speeds than when higher VSLs were posted with longer distances between gantries. These higher levels of compliance at low VSLs likely indicate that traffic was restricting the ability to travel above the posted VSL. The results comparing speeds upstream of gantries showed a similar pattern based on the speed posted, but no pattern was observed with respect to compliance over space. This study may be useful to transportation and law enforcement agencies in determining when to perform targeted enforcement and in planning for future deployments.