A Comparison of Floating Car vs. Loop Detector Estimated Freeway Travel Time Delay

Abstract

ABSTRACT Floating car- and loop detector-based methods are two different types of methods frequently used to collect travel time delay information across a freeway network. Sometimes, it is necessary to use them jointly to achieve the necessary freeway network coverage, due to the high labor costs for the floating car-based method and the indispensability of sufficient network instrumentation for the loop detector-based method. For example, both floating car- and loop detector-based methods were once used in the Highway Congestion Monitoring Program in the California Department of Transportation. It is therefore necessary to evaluate whether these two types of methods estimate similarly in terms of total travel time delay. To this end, corresponding delay information estimated using both types of methods from 37 freeway segments in the Greater Sacramento Area were collected and compared. It was found that these two types of methods do not estimate similarly in terms of total segment travel time delay. The mean absolute relative difference (MARD) can be as high as 78%, especially when delay is defined using a lower reference speed, such as 56 km/h. However, in terms of total segment travel time, the loop detector and the modified floating car method estimated similarly. The MARD is 19%. It was also found that the estimation from the different methods did correlate fairly well, which provides a means of conversion when different methods are used to monitor the total delay across a freeway network. As a spin-off, it was also found that a 1.5 km spacing of loop detectors is sufficient to achieve the 19% MARD as compared with the modified floating car method in terms of total travel time estimation.