Evaluating Effectiveness of Dynamic Speed Display Signs in Transition Zones of Two-Lane Rural Highways in Pennsylvania

Although law enforcement is an effective method to manage speeds, it requires significant resources to ensure adequate spatial and temporal compliance. In some cases, alternative speed management methods may be necessary to deter drivers that travel in excess of the posted speed limit. One such method is via the use of dynamic speed display signs that serve to help motorists “self-enforce” their speed. Dynamic speed display signs measure the speed of approaching vehicles and communicate the speed to drivers on a digital display. The Pennsylvania Department of Transportation has invested in several portable dynamic speed display signs and selected several locations to implement them along two-lane rural highway transition zones. Transition zones are longitudinal roadway sections that contain a high-speed segment followed by a low-speed segment and are commonly encountered in Pennsylvania along two-lane rural highways that pass through rural communities. A before-during-after observational study of free-flow passenger car operating speeds was undertaken at 12 transition zones to determine the effectiveness of the dynamic speed display signs. The results of the analyses indicate that the dynamic speed display signs are effective in reducing free-flow speeds by an average of 6.4 mph (10.3 km/hr) while in place and activated. However, observed operating speeds increased by an average of 6.6 mph (10.6 km/hr) after the devices were removed from the study sites. Linear regression models were developed to predict the speed reductions expected along transition zones where dynamic speed display signs are contemplated, and to predict the speed increases that are expected after the signs are removed from the transition zones.