Impacts of Sensor Spacing on Accurate Freeway Travel Time Estimation for Traveler Information

Travel-time estimation is a critical ingredient for transportation management and traveler informationboth infrastructure-based and in-vehicle. Infrastructure managers and operators are interested in estimating optimal freeway sensor density for new construction and retrofits. Focusing on freeway travel time estimation for display on roadside variable message signs, this article describes a concept developed from first principles of traffic flow for establishing optimal sensor density. The method is based on computing the magnitude of under- and overprediction of travel time during shock passages when using the midpoint method. Two case studies are presented considering representative traffic dynamics situations. Along with other performance metrics, a suggested aggregate measure developed from vehicle-hours traveled (VHT) is described for a reasonable range of detector densities. Extensions of the method to account for both recurrent and nonrecurrent congestion are included. Finally some suggestions for future research are described.