Speed limits are generally created for ideal road conditions. When road conditions deteriorate, speed limits tend to become unreasonable, and enforcement of safe driving becomes difficult. Variable speed limit (VSL) systems are used to adjust speeds in accordance with the changing conditions in an attempt to reduce the variation of speeds among all vehicles. This research highlights the findings from the variable speed limit system in place along the Elk Mountain Corridor of I-80 in Southeastern Wyoming during the first and second full winter seasons of VSL implementation. Specifically this research focuses on speeds and variations of speed and the effect the VSL system has had on individual vehicles. The objective of this research was to determine if the VSL system was effective in reducing speeds and speed variation during adverse conditions. Further, this analysis attempts to determine the effect of the VSL system on driver behavior. The VSL system was effective at reducing speeds when properly implemented, reduced speed variation as well. Drivers respond differently to different levels of VSL implementation.

Speed, speed variation, and speed compliance are important factors related to both frequency and severity of traffic crashes. Decreasing the speed variation and increasing the driver compliance levels have been realized as effective methods to improve the traffic safety. Variable speed limits (VSL) systems along rural roadways are an operational strategy used to address speed related safety problems. VSL systems can aid drivers in selecting safe speeds for the current conditions, which is critical on Wyoming roadways where frequent and severe winter weather conditions often occur. To ensure that the VSL systems will be effective, the posted variable speeds need the highest degree of speed compliance from drivers in the corridor. The objective of this research is to determine the effects of the VSL system on driver's speed behavior on rural corridors and provide the guidance for further development of the VSL system in Wyoming. Results showed that the VSL systems were effective in reducing the speed and speed variation as well as increasing the driver compliance during the implementation period. However, drivers showed the different speed responses according to the various VSL implementation levels. Drivers showed a low tolerance for a large speed reduction on the VSL.

Low visibility conditions can inhibit a driver's ability to perceive appropriate operating speeds, particularly during foggy conditions in which the characteristics of the fog can vary spatiotemporally. By reducing visibility and contrast in the visual field, fog obscures crucial driving cues essential for perceiving depth and speed. Studies have shown that fog-related crashes are more likely to involve multiple vehicles and severe injuries than crashes during clear conditions. Numerous agencies have installed countermeasures such as weather advisory systems and variable speed limits (VSLs) to mitigate these conditions, but not many studies have quantitatively analyzed the results of these countermeasures. In October 2016, the Virginia Department of Transportation (VDOT) activated a VSL system on a 12-mile section of I-77 that runs through mountainous terrain in southwestern Virginia. The area is known to have severe, recurring fog events, so the VSL system was installed to reduce the quantity and severity of crashes in the corridor. This study assessed how the I-77 VSL system has affected speeds and crash characteristics since its activation. Before the installation of the VSL, drivers frequently drove much faster than the safe speed based on the stopping sight distance during fog. The purpose of the VSL system was to influence drivers to travel closer to the safe speed during reduced visibility events by posting appropriate reduced speed limits. The analysis examined the effect of the VSL system on driver speeds before and after activation at a single site and after activation across the corridor. Effects on crashes for the entire corridor were also examined. The results showed statistically significant reductions in mean speeds and variances after the VSL was activated, and drivers drove closer to the safe speed based on available visibility. Models developed to understand how the VSL system affected speed as a function of visibility showed that speeds were reduced by a statistically significant amount when VSLs were active. Trends in speed by posted speed limit were examined across the corridor, and it was found that compliance generally improved once drivers encountered reduced visibilities. Speeds did not change as much in transition areas leading into the area where the fog was present, however. Crash analysis revealed only two fog-related crashes in the after period, yielding reduced crash rates during low visibility conditions and indicating improved safety. These safety results are considered preliminary, however, because of limited after data. The results of this VSL implementation may be used to refine the current VSL control algorithm to improve compliance even further and could also serve as a reference for other agencies contemplating alternatives to improve safety at fog-prone areas. Given the results, it is recommended that VDOT's Southwest Region Operations convene a group to modify the VSL control algorithm. It is further recommended that the Virginia Transportation Research Council re-evaluate the safety effects of the system after at least 3 years of after data are available to make a more definitive determination of the safety effects of the system and to determine its return on investment. Implementation of these recommendations could further improve the efficacy of the system and result in a better quantification of the full benefits of the system.

TRB Special Report 254 - Managing Speed: Review of Current Practices for Setting and Enforcing Speed Limits reviews practices for setting and enforcing speed limits on all types of roads and provides guidance to state and local governments on appropriate methods of setting speed limits and related enforcement strategies. Following an executive summary, the report is presented in six chapters and five appendices.

Variable Speed Limit (VSL) systems are an innovative way of managing traffic using Intelligent Transportation Systems (ITS). Unlike the traditional static speed limit signs, VSLs are speed limit signs with a capability of changing regulatory speed limits according to changing real time weather and traffic conditions. VSLs are primarily used to reduce traffic congestion and improve safety along roadways. Wyoming weather is highly variable and often severe, in the absence of recommended speed limits for such conditions drivers select their own safe driving speed leading to high variance among vehicles. This high speed variance, results in high crash rates and frequent road closures. Currently Wyoming Department of Transportation (WYDOT) using an interim manual protocol to deploy speed limits on VSL corridors. A new automated self-learning methodology for deploying VSL is described in this dissertation. As a first step in this project, weather variables that have a significant effect on vehicle speeds during bad weather conditions were identified using a robust regression statistical procedure. Second, an automated control strategy was designed that uses real time weather and observed speed variables. Third, a statistical model called decision trees was used in control strategy to enhance the performance of the automated control strategy. Fourth, a self-learning component is added to the control strategy to address concerns about the high variability of different storm events in this region. The proposed automated self-learning algorithm was tested on previous storm event datasets and results from the simulation indicate that the new system is more efficient than the current manual protocol of deploying speed limits. Finally, the transferability of the proposed control strategy was evaluated by applying the self-learning control strategy that was developed based on the storm data available at the Elk Mountain corridor to the storm data available at Laramie-Cheyenne corridor.