Evaluation of Offline and Online Speed-based Travel Time Estimation on Expressway

Abstract

The objective of this paper is to evaluate travel time estimation techniques for off-line and on-line applications using traffic speed. The study area was the 8.1-km Chalerm Mahanakorn Expressway Daokanong - Port section in Bangkok. Traffic speeds were collected at 7 stations. Average traffic volume of 6,140 vph (vehicles per hour) was observed during the 6:00am to 22:00pm period. Four offline methods were used to convert speed into travel time for each link, namely: mid-point, section average, weighted section average, and San Antonio's (lower speed). To aggregate link travel time into route travel time, the instantaneous and time slice methods were considered. The estimated travel times were then compared with the actual travel time observed from 1,632 vehicles. The results indicate that the accuracy depends on the time period and level of traffic congestion. Considering route travel time, the time slice method gives a slightly better accuracy than the instantaneous method. Comparing the offline (all data present) and online (short-term forecast of speed in the next time slice(s)) travel time estimation, the two methods yield similar accuracy. The findings of the research imply that the practical travel time estimation may need several methods suitable for each time period. The improved estimation can increase the accuracy over a single method, thereby decreasing the MAPE (mean absolute percentage error) from 14-20% to 11%.