Stream travel time reliability using GPS-equipped probe vehicles

Travel time reliability (TTR) is an important measure to quantify the variation in travel times. Currently, there is no single reliability metric appropriate across all locations, that is easily understandable and can be used to compare across facilities. Moreover, reliability analysis of facilities from developing countries is limited due to the non-availability of extensive data required for such an analysis. The present study addresses these gaps. It identifies a reliable data source for such analysis of heterogeneous, lane-less traffic, compares existing reliability measures for the data, highlights the advantages and disadvantages, proposes a measure that may be more suitable for such traffic with high variability, and finally illustrates how reliability analysis under such conditions can be done with limited data sources such as GPS-fitted transit vehicles. Using such commonly available data for traffic stream reliability analysis is the ultimate aim of this study. For validation, stream travel time from Wi-Fi scanners is used. The study analyses the performance of various reliability measures and identifies the most suitable ones. Following this, a reliability measure, i.e. capacity buffer index (CBI), is developed to identify the unreliable congested regimes or periods, keeping time taken to travel at capacity conditions as the benchmark. From the results, it has been observed that CBI is in agreement with the real-field conditions in 94% of the cases, whereas it is 75% buffer time index. Finally, the feasibility of using bus probes to measure stream TTR is checked. Results show that bus probes can be an indicator of stream reliability and the developed measure can effectively capture the relationship between stream and bus TTR