Impact of Heavy Vehicles on Highway Traffic Flows: Case Study in the Seoul Metropolitan Area

Highway capacity manuals (HCM) have incorporated the effect of heavy vehicles (HVs) in the form of passenger car equivalents (PCEs) for estimating highway traffic conditions, which include the road’s slopes, the length of the road, the geographical characteristics, and the ratio of HVs to other types of vehicles on the road. The PCEs of HVs are critical for designing, operating, and evaluating highways. In order to analyze the effects of HVs on highway traffic flow, the real-time automatic vehicle classification (AVC) data were collected for three different major highways (four-lane, six-lane, and eight-lane highways, respectively) in the Seoul metropolitan area for the 2 months from August to September 2011. The purpose of this study was to examine the effect of HVs on traffic flows using real-time AVC data and to analyze the relationship among the average speed, the HV ratio, the flow rate, and the number of lanes. The findings revealed that the average speed decreased when the flow rate and HV ratio increased for the six-lane and the eight-lane highways. The average speed also decreased as the flow rate increased for the four-lane highway. However, the average speed stabilized when the HV ratio was in the range of 25–45% and recovered when the HV ratio exceeded 45%; this result implied a need to reexamine the existing methods of deriving the HV factors for the HCM.