Weather responsive variable speed limit systems
Weather conditions have significant impact on the safety and operations of the highway transportation system. Rain, snow and ice can reduce pavement friction and increase the potential for crashes especially when vehicles are traveling too fast for conditions. Under these circumstances, the posted speed limit at a location may no longer be safe and appropriate. Inclement weather can also have considerable impacts on the operations of highways, lowering the capacity of highway system and decreasing the efficiency of the system for drivers. Consequently, new approaches are necessary to influence motorists' behavior in regards to speed selection when inclement weather presents the potential for reduced pavement friction at a given location. Among these approaches is the use of weather responsive variable speed limit (VSL) systems. This thesis reviews the current state of practice of weather responsive VSL systems and other similar systems. It also characterizes the problems faced at a potential weather responsive VSL system location through the analysis of crash, speed and weather data. This effort also includes the concept development of a system for the proposed location. A critical component of these systems (the non-invasive weather sensor) is also evaluated to determine its capabilities for use in these and similar systems. Current practice showed the use of weather responsive VSL systems for rain, snow, ice, fog, and wind. In general, these systems were found to have positive effects in reducing crashes and speeds. The proposed study site experienced crashes at a rate higher than expected for similar locations. Also over 60% of crashes at the location occur during wet pavement conditions, but the pavement at the site is only wet approximately 6% of the time. Speed data analysis shows that drivers at this location don't reduce their speeds much during wet conditions. A system concept for the proposed location is presented. The sensor evaluation determined that the sensor is capable of producing valuable information for VSL and similar systems. A calibration is also evaluated and proven to greatly improve the accuracy of the water depth measurements produced by the sensor.