Scholarship & Research
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Item Empirical assessment of a congestion and weather-responsive advisory variable speed limit system(Montana State University - Bozeman, College of Engineering, 2016) Siddiqui, Sohrab; Chairperson, Graduate Committee: Ahmed Al-KaisyTraffic congestion and safety along urban corridors have become major challenges for most highway agencies in the United States. Adverse weather conditions also present a considerable challenge, both in terms of safety and operations. All these problems along with the increasingly limited resources for infrastructure expansion have urged transportation agencies to investigate innovative traffic management approaches. One of these approaches is the use of Active Traffic Management (ATM) strategies. Within ATM, the practice of Variable Speed Limit (VSL) systems is well suited to improving safety and operations. These systems dynamically utilize real-time traffic and/or weather data to post appropriate speeds that are thought to improve safety and operations along a corridor. The overall aim of this thesis is to investigate the benefits of a recently installed advisory VSL system along OR-217 freeway in Portland, Oregon. This corridor is characterized by high traffic levels, severe congestion and unreliable travel times. The congestion of the freeway contribute to crash rates exceeding the statewide averages for this type of facility. Pacific Northwest's unpredictable climate presents another challenge that doubles the congestion and safety problems along the corridor. The effectiveness of this system was explored through an in-depth 'before and after' and 'on-and-off' analyses. The study was designed in a way that it encompasses both the safety and mobility benefits of the system. Besides, driver compliance with the system was also measured under different scenarios. The results indicated that the system had significant impacts on both mobility and safety. In terms of mobility it was found that system had lowered the average speeds along the corridor. The advisory VSL activation also resulted in reduced capacities. Safety assessment of the system suggested that, VSL has decreased crash rates and temporal and lateral variations of speed. Under certain scenarios, the system also decreased the longitudinal variations of speed. Further, it was also found that due to the advisory nature of the system, the majority of drivers do not comply with the system. However, VSL has resulted in reducing the percentage of aggressive drivers and have increased the number of drivers complying the static speed limit.Item Weather responsive variable speed limit systems(Montana State University - Bozeman, College of Engineering, 2013) Ewan, Levi Austin; Chairperson, Graduate Committee: Patrick McGowenWeather 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.