4. Design of Spot Speed Methods for Real-Time Provision of Traffic Information

The accuracy of travel time information disseminated in real-time on metropolitan freeways is one of the key issues in the development of advanced traveler information systems. It is generally considered that travel time estimations based on spot speed measurements at loop detectors are not accurate enough to support this real-time information need. This has brought traffic agencies to the view that for real-time information systems to be effective they would need to have much more accurate ways of measuring travel times. Many fancy technologies to directly measure vehicular travel times are being proposed. This chapter shows that, in the real-time context, the precision of the system is not related solely to the accuracy of the measurement. Immediacy and forecasting capabilities play a role. Therefore, focusing only on the accuracy of the travel time measurement is a myopic approach, which can lead to counterintuitive results. Specifically, it is claimed that using travel times estimated with the traditional spot speed Midpoint Algorithm, the performance of the real-time information system is better than by using much more accurate directly measured travel times. Guidelines for an adequate configuration of the common parameters of the system are provided. In addition, real-time context enhancements for travel time estimation methods based on punctual speed measurements are proposed. These are addressed by taking into account an easy and practical implementation. They have been proven to work well in an empirical application on a Spanish Freeway.