Evaluating effectiveness of dynamic speed display signs in transition zones of two-lane, rural highways in Pennsylvania

Although law enforcement is an effective way 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 who travel faster than the posted speed limit. One such method is through the use of dynamic speed display signs (DSDSs) that help motorists self-enforce their speed. DSDSs measure the speed of approaching vehicles and communicate the speed to drivers on a digital display. The Pennsylvania Department of Transportation invested in several portable DSDSs and selected several locations to use them along two-lane, rural highway transition zones. Transition zones are longitudinal roadway sections that contain changes in the posted speed limit; they are commonly encountered in Pennsylvania where high-speed, two-lane highways pass through rural communities. The section of highway that passes through the rural community has a lower posted speed limit than the upstream, high-speed section of the highway. A before, during, and after observational study of free-flow vehicle operating speeds was undertaken at 12 transition zones to determine the effectiveness of DSDSs. The results of the analyses indicate that the DSDSs are effective in reducing free-flow passenger car operating speeds by an average of 6 mph (10 km/h) while in place and activated. However, the speed reductions observed while the DSDSs were in place faded after the devices were removed from the study sites.