A Nine-Month Before and After State Study Showed That Variable Speed Limit System Increased Safety on a Freeway in Virginia During Congested Conditions.
Fredericksburg, Virginia
Performance Evaluation Application for the I-95 Variable Speed Limit System
Summary Information
Variable Speed Limit (VSL) systems are implemented to enhance safety and mitigate traffic congestion. These systems employ dynamic signs that adjust speed limits in response to real-time traffic conditions, replacing conventional static speed limit signs. In June 2022, Virginia Department of Transportation (VDOT) activated a VSL system on a 15-mile section of I-95 Northbound south of Fredericksburg, a corridor that experienced significant recurring and non-recurring congestion, especially in the summer months. The VSL system utilized a proprietary algorithm that used data from traffic detectors to determine a speed limit to optimize traffic flow. The objective of this study was to conduct an evaluation of the VSL system’s operations and safety.
METHODOLOGY
The VSL system included 24 detector stations that generated per-vehicle record (PVR) data. These detectors produced raw PVR data which included unique vehicle ID, time, detector station ID, lane, speed, etc. The VSL system's algorithm produced posted speed limit (PSL) data that had time stamp, location, and VSL recommended speed. The performance measures used for evaluation included: (i) speed compliance (change in mean speed, speed compliance, etc.), (ii) operations (change in delay, travel time reliability, etc.), and (iii) crashes (change in crash frequencies and collision type by severity and change in secondary crashes). The study compared crash and speed data from June 22, 2021, to March 31, 2022, prior to the system activation with the same type of data from June 22, 2022 to March 31, 2023.
FINDINGS
- Fatal and serious crashes were reduced by 13 percent compared to the period before the VSL system activation.
- Rear end crashes dropped by six percent compared to the period prior to system activation.
- Same direction side-swipe crashes were reduced by 13 percent compared to the prior period.
- Speeds before and after VSL activation were compared by filtering for similar conditions. The results showed that when the 55MPH speed limit was active, there were 22 percent fewer vehicles traveling 10+MPH over the speed limit.
