Variable Speed Limit Sign (VSLS) systems enable transportation managers to dynamically change the posted speed limit in response to prevailing traffic and/or weather conditions. VSLS are thought to improve safety and reduce driver stress while improving traffic flow and travel times. Although VSLS have been implemented in a limited number of jurisdictions throughout the world, there is currently very limited documentation describing the quantitative safety and operational impacts. The impacts that have been reported are primarily from systems in Europe, and may not be directly transferable to other jurisdictions, such as North America. Furthermore, although a number of modelling studies have been performed to date that quantify the impacts of VSLS, the VSLS control strategies are often too complex or based on unrealistic assumptions and therefore cannot be directly applied for practical applications. Consequently, a need exists for an evaluation framework that quantifies the safety and traffic performance impacts of comprehensive VSLS control strategies suitable for practical applications in North America. This paper presents the results of an evaluation of a candidate VSLS system for an urban freeway in Toronto, Canada. The evaluation was conducted using a microscopic simulation model (i. e. a model that predicts individual vehicle movements) combined with a categorical crash potential model for estimating safety impacts. <br /><br /> The objectives of this thesis are: 1) to validate a real-time crash prediction model for a candidate section of freeway; 2) to develop a candidate VSLS control algorithm with potential for practical applications; 3) to evaluate the performance of the VSLS control strategy for a range of traffic conditions in terms of safety and travel time; and 4) to test the sensitivity of the VSLS impact results to modifications of the control algorithm. <br /><br /> The analysis of the VSLS impacts under varying levels of traffic congestion indicated that the candidate control strategy was able to provide large safety benefits without a significant travel time penalty, but only for a limited range of traffic conditions. The tested algorithm was found to be insufficiently robust to operate effectively over a wide range of traffic conditions. However, by modifying parameters of the control algorithm, preliminary analysis identified potential improvements in the performance of the VSLS. The modified control strategy resulted in less overall travel time penalty without an adverse impact on the safety benefits. It is anticipated that further modifications to the VSLS control strategy could result in a VSLS that is able to operate over a wide range of traffic conditions and provide more consistent safety and travel time benefits, and it is recommended that the framework used in this study is an effective tool for optimizing the algorithm structure and parameter values.

Variable Speed Limit Sign (VSLS) systems enable transportation managers to dynamically change the posted speed limit in response to prevailing traffic and/or weather conditions. VSLS are thought to improve safety and reduce driver stress while improving traffic flow and travel times. Although VSLS have been implemented in a limited number of jurisdictions throughout the world, there is currently very limited documentation describing the quantitative safety and operational impacts. The impacts that have been reported are primarily from systems in Europe, and may not be directly transferable to other jurisdictions, such as North America. Furthermore, although a number of modelling studies have been performed to date that quantify the impacts of VSLS, the VSLS control strategies are often too complex or based on unrealistic assumptions and therefore cannot be directly applied for practical applications. Consequently, a need exists for an evaluation framework that quantifies the safety and traffic performance impacts of comprehensive VSLS control strategies suitable for practical applications in North America. This paper presents the results of an evaluation of a candidate VSLS system for an urban freeway in Toronto, Canada. The evaluation was conducted using a microscopic simulation model (i. e. a model that predicts individual vehicle movements) combined with a categorical crash potential model for estimating safety impacts. <br /><br /> The objectives of this thesis are: 1) to validate a real-time crash prediction model for a candidate section of freeway; 2) to develop a candidate VSLS control algorithm with potential for practical applications; 3) to evaluate the performance of the VSLS control strategy for a range of traffic conditions in terms of safety and travel time; and 4) to test the sensitivity of the VSLS impact results to modifications of the control algorithm. <br /><br /> The analysis of the VSLS impacts under varying levels of traffic congestion indicated that the candidate control strategy was able to provide large safety benefits without a significant travel time penalty, but only for a limited range of traffic conditions. The tested algorithm was found to be insufficiently robust to operate effectively over a wide range of traffic conditions. However, by modifying parameters of the control algorithm, preliminary analysis identified potential improvements in the performance of the VSLS. The modified control strategy resulted in less overall travel time penalty without an adverse impact on the safety benefits. It is anticipated that further modifications to the VSLS control strategy could result in a VSLS that is able to operate over a wide range of traffic conditions and provide more consistent safety and travel time benefits, and it is recommended that the framework used in this study is an effective tool for optimizing the algorithm structure and parameter values.

Recent research at the University of Central Florida involving crashes on Interstate-4 in Orlando, Florida has led to the creation of new statistical models capable of determining the crash risk on the freeway (Abdel-Aty et al., 2004; 2005, Pande and Abdel-Aty, 2006). These models are able to calculate the rear-end and lane-change crash risks along the freeway in real-time through the use of static information at various locations along the freeway as well as the real-time traffic data obtained by loop detectors. Since these models use real-time traffic data, they are capable of calculating rear-end and lane-change crash risk values as the traffic flow conditions are changing on the freeway. The objective of this study is to examine the potential benefits of variable speed limit implementation techniques for reducing the crash risk along the freeway. Variable speed limits is an ITS strategy that is typically used upstream of a queue in order to reduce the effects of congestion. By lowering the speeds of the vehicles approaching a queue, more time is given for the queue to dissipate from the front before it continues to grow from the back. This study uses variable speed limit strategies in a corridor-wide attempt to reduce rear-end and lane-change crash risks where speed differences between upstream and downstream vehicles are high. The idea of homogeneous speed zones was also introduced in this study to determine the distance over which variable speed limits should be implemented from a station of interest. This is unique since it is the first time a dynamic distance has been considered for variable speed limit implementation. Several VSL strategies were found to successfully reduce the rear-end and lane-change crash risks at low-volume traffic conditions (60% and 80% loading conditions). In every case, the most successful treatments involved the lowering of upstream speed limits by 5 mph and the raising of downstream speed limits by 5 mph. In the free-flow condition (60% loading), the best treatments involved the more liberal threshold for defining homogeneous speed zones (5 mph) and the more liberal implementation distance (entire speed zone), as well as a minimum time period of 10 minutes. This treatment was actually shown to significantly reduce the network travel time by 0.8%. It was also shown that this particular implementation strategy (lowering upstream, raising downstream) is wholly resistant to the effects of crash migration in the 60% loading scenario. In the condition approaching congestion (80% loading), the best treatment again involved the more liberal threshold for homogeneous speed zones (5 mph), yet the more conservative implementation distance (half the speed zone), along with a minimum time period of 5 minutes. This particular treatment arose as the best due to its unique capability to resist the increasing effects of crash migration in the 80% loading scenario. It was shown that the treatments implementing over half the speed zone were more robust against crash migration than other treatments. The best treatment exemplified the greatest benefit in reduced sections and the greatest resistance to crash migration in other sections. In the 80% loading scenario, the best treatment increased the network travel time by less than 0.4%, which is deemed acceptable. No treatment was found to successfully reduce the rear-end and lane-change crash risks in the congested traffic condition (90% loading). This is attributed to the fact that, in the congested state, the speed of vehicles is subject to the surrounding traffic conditions and not to the posted speed limit. Therefore, changing the posted speed limit does not affect the speed of vehicles in a desirable manner. These conclusions agree with Dilmore (2005).