Development And Testing Of Variable Speed Limit Control Logics For Work Zones Using Simulation

Development and Testing of Variable Speed Limit Control Logics for Work Zones Using Simulation

There has been a considerable increase in the amount of construction work on the U.S. national highways in recent years. This is a direct result of the current transportation policies emphasis on the maintenance of existing facilities rather than on building new ones. But with the increase in the amount of traffic on the interstates, the safety at the construction sites is getting worsened. Variable speed limit (VSL) systems are a kind of intelligent transportation systems (ITS) technology and they try to increase safety by posting the maximum or minimum speed limits. The speeds to be posted on the VSL signs are calculated manually or with the help of algorithms. This study is an attempt to evaluate the performance of various speeds at work zones and to develop VSL control logics that would calculate suitable speeds for different traffic conditions. The research is conducted by simulating a postulated test-bed network and then validating the results by simulating a real world work zone site. The study uses a microscopic simulation model - VISSIM. The study developed a safety surrogate measure, minimum safety distance equation (MSDE), to quantify safety. Travel times have primarily been used to quantify mobility. Since most of the existing VSL control logics consider only the traffic advisory speed or average traffic speed, an attempt was made to develop a logic that would consider both safety and mobility in calculating speeds. The results of simulating the postulated network indicated that the performance of speeds varied with traffic demand volumes and compliance rate conditions. In general an increase in speed decreased safety, but improved travel time. Surprisingly at low volume and low compliance conditions an increase in speed also improved safety. In order to find the speeds that provide optimal measures of safety and mobility a normalization procedure that combines travel time and safety measures is used. The performance of the logics is mixed. In general logic 3 (that considers both safety and mobility measures to calculate VSL speed) performed better than logics 1 and 2 (that use average traffic speed). However logics 1 and 2 performed better than Logic 3 for oversaturated conditions and for low volume and low compliance conditions. Statistical analysis was performed to substantiate the results of the base cases and the logics. The performance of the VSL system was also evaluated by simulating a real world work zone site at Covington, Virginia. The results were found to be consistent with those of the postulated network.

Evaluation of Variable Speed Limits in Work Zones

A Highway Work Zone Design and Traffic Management Decision System