Using Floating Cars to Measure Travel Time Delay: How Accurate Is the Method?

Abstract

In practice, partial floating car runs (runs in the direction of the traffic flow of interest) plus a measured or assumed controlling bottleneck capacity could be used to estimate the total travel time delay along a freeway segment. The floating car's travel time in excess of that of user-specifiedfree flow travel is defined as the floating car's measured travel time delay. This delay, along with the bottleneck capacity, could be used to estimate excess traffic demand. The total freeway segment delay could then be determined on the basis of the fundamental queuing diagram. To examine the quality of this estimation method, an explicit evaluation of the accuracy level of the estimated delay was conducted. Trajectory data for thousands of vehicles on six freeway facilities across the United States were analyzed, and a floating car's travel time and delay were generated (simulated) as the average travel time and delay of all its neighboring vehicles within a specified duration. This was found to be an inaccurate method. Although performance is influenced by the combination of such factors as the user-specified free flow speed (delay definition), the floating car intervals, the floating car generation method, and facility-specific traffic conditions, the assumed bottleneck capacity is observed to be the primary contributor to inaccuracy. Until a practicable method is found for determining the true flow between any two consecutive floating cars to supersede the assumed capacity, the estimation should be interpreted only in its comparative sense, such as tracing the trend of delay development for a freeway facility.