Evaluation of the Highway Capacity Manual (HCM) and Thailand’s Department of Highways’ approaches to estimating the capacity of a multilane highway segment via the empirical method

Abstract

This study aims to evaluate the methods used by the Thailand Department of Highways (DOH) and the Highway Capacity Manual (HCM 2010 and HCM 2016) for estimating the capacity of an urban multilane highway segment in Thailand, by comparing these estimates with the empirical capacity derived from a speed-flow plot. Field data were collected from a six-lane urban highway segment in Thailand, followed by documentation of the roadway conditions and analysis of vehicle composition. Capacity estimates were calculated using the DOH and HCM methods prior to comparing them to the empirically measured capacity. The results showed that the empirical capacity was 1,619 pc/h/ln. The capacities estimated using the HCM 2010 and HCM 2016 methods were 34.8% and 35.3% greater than the empirically measured capacity, respectively, while the capacity estimated using the DOH method was 14.1% lower than the measured capacity. These findings indicate significant discrepancies in the estimates produced by the three models, when applied to the multilane highways segment. Given that the DOH model has not been updated in over two decades, this study concludes that its accuracy in capacity estimation may be compromised by evolving factors, such as driver behavior, traffic flow characteristics, vehicle types, and vehicle performance. Moreover, the DOH method's omission of variables (including access point density, median type, and terrain type) may further affect its accuracy. Similarly, since HCM 2010 and HCM 2016 are tailored to the United States, using both models to estimate the capacity of the multilane highway segment of interest may lead to errors due to differences in driver behaviors between Thai and American drivers, distinct traffic flow characteristics, the model’s omission of the percentage of motorcycles, and the overestimation of speed at capacity on the highway segment.