A small-scale study of the safety impacts of end-of-queue warning systems at work zones.
An innovative end-of-queue warning system was implemented on a 96-mile section of Interstate 35 (I-35) in central Texas as part of a freeway widening project. In general, the system was designed to alert motorists of slowed or stopped vehicles ahead of them as they approached active construction projects on I-35. These work zones required lane closures and merging at nighttime in rural areas where drivers would not expect sudden slowdowns.
The system used portable roadside radar detection devices mounted upstream of work zones and lane closures to monitor and measure the speeds of approaching vehicles. An algorithm was used to analyze the detector data and select a warning message on portable roadside dynamic message signs installed further upstream. The expected volume and capacity reduction from a work zone database were used to estimate the maximum expected queue for the night in order to guide placement of the sign. The warning message informed drivers of the work zone and approximate distance to the slowed or stopped traffic, enabling traffic to gradually reduce speed as they approached the end of a vehicle queue.
Based on a small-scale study with limited duration, preliminary evaluation results indicated that the end-of-queue warning system reduced crash potential by 18 to 45 percent. With the end-of-queue warning system, the proportion of lane closure crashes with high severity (injury or fatal crash) was estimated to be 41 percent vs. 58 percent without the system.
The deployment over 200 nighttime lane closures was estimated to have saved between $1.4 million and $1.8 million in societal costs associated with crashes, at a rate of $6,600 to $10,000 in saved societal costs per night of deployment.