Understanding the Highway Safety Benefits of Different Approaches of Connected Vehicles in Reduced Visibility Conditions
This study evaluated the effectiveness of connected vehicle (CV) technologies in adverse visibility conditions using microscopic traffic simulation. Traffic flow characteristics deteriorate significantly in reduced visibility conditions resulting in high crash risks. This study applied CV technologies on a segment of Interstate I-4 in Florida to improve traffic safety under fog conditions. Two types of CV approaches (i.e., connected vehicles without platooning (CVWPL) and connected vehicles with platooning (CVPL) were applied to reduce the crash risk in terms of three surrogate measures of safety: the standard deviation of speed, the standard deviation of headway, and rear-end crash risk index (RCRI). This study implemented vehicle-to-vehicle (V2V) communication technologies of CVs to acquire real-time traffic data using the microsimulation software VISSIM. A car-following model for both CV approaches was used with an assumption that the CVs would follow this car-following behavior in fog conditions. The model performances were evaluated under different CV market penetration rates (MPRs). The results showed that both CV approaches improved safety significantly in fog conditions as MPRs increase. To be more specific, the minimum MPR should be 30% to provide significant safety benefits in terms of surrogate measures of safety for both CV approaches over the base scenario (non-CV scenario). In terms of surrogate safety measures, CVPL significantly outperformed CVWPL when MPRs were equal to or higher than 50%. The results also indicated a significant improvement in the traffic operation characteristics in terms of average speed.
It is known that reduced visibility due to fog has caused serious traffic safety and flow issues. Florida had experienced a total of 4,954 fog-related crashes between the year of 2008 and 2012, of which 132 crashes were fatal, and about 30% of the total fog-related crashes were fatal and injury crashes. It is worth mentioning that fog-related crashes tend to result in more severe injuries and involve more vehicles compared with clear conditions crashes. Fog affects roadway safety by increasing crash risk. Therefore, it is necessary to evaluate the appropriate countermeasures to enhance traffic safety under fog conditions.
There has already been a lot of research conducted on traffic safety under normal weather conditions. On the other hand, traffic safety under fog conditions has attracted much less attention. However, some researchers have already proposed the traditional approach of variable speed limits (VSL) or variable message signs (VMS) to decrease the crash risk in reduced visibility conditions. This approach can possibly improve traffic safety and mitigate traffic crashes by adjusting vehicle speeds and decreasing speed variation among vehicles in reduced visibility conditions. Nevertheless, the success of VSL or VMS is dependent on the level of compliance. Therefore, the VSL would not guarantee the improvement of traffic safety if drivers do not follow the new speed limit.
An innovative feature of this study was to apply the connected vehicle (CV) technologies in adverse visibility conditions under a microsimulation environment. To be more specific, this research aims to contribute to the implementation of two CV approaches, that is, connected vehicle without platooning (CVWPL) and connected vehicle with platooning (CVPL), to improve traffic safety in reduced visibility conditions. CVPL concept is an extension of CVWPL approach in which several CVs form a “platoon” that behaves as a single unit. A car-following model for both CV approaches was also used in fog conditions with an assumption that applied CVs would follow this car-following behavior in the simulation. The most significant difference of CVs driving behavior between the two approaches was joining vehicles to maintain a platoon. In the near future, the market penetration rate (MPR) will not achieve 100%. Meanwhile, the penetration will increase gradually. Hence, it is worthwhile to study the safety benefits of CV technologies under different MPRs.
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