The California Department of Transportation (Caltrans) freeway sensor network has two components: the sensor system of 25,000 inductive loop sensors grouped into 8,000 vehicle detector stations (VDS) and covering 30,500 freeway direction-miles; and the communication network over which the sensor measurements are transported to Caltrans Traffic Management Centers. Thesensor network is virtually the only source of data for use in traffic operations, performance measurement, planning and traveler information. However, the value of these data is greatly reduced by the poor reliability of the sensor network: On a typical day in 2005, only 60 percent of the statewide sensor network provided reliable measurements. This is an empirical study of the reliability of the sensor network based on two data sets. The first set, obtained from PeMS, consists of a daily summary of the quality of data received from each loop sensor in Caltrans Districts 4, 7 and 11. The second data set consists of reports of field inspections of morethan 4,000 loops each in Districts 4 and 7 part of Caltrans' Detector Fitness Program. The paper proposes and calculates four non-parametric metrics of system performance: productivity, stability and lifetime and fixing time. Based on these metrics the authors compare the performance of the different districts and the limited effectiveness of the field inspections. The authors conclude that the performance of a sensor network has two independent dimensions: the communication network and the sensing network. Any overall performance gain requires that both dimensions be improved.
Samenvatting