Road investment appraisal models have had difficulty incorporating the effects of congestion on vehicle fuel consumption. The new Highway Development and Management Model, HDM-4, incorporates the Australian Road Research Board's ARFCOM model to predict fuel consumption in combination with a simulated vehicle speed profile. A key determinant of the simulated speed profile is acceleration noise (the standard deviation of the second-by-second acceleration) which is in turn derived from the highway volume capacity ratio. To investigate this further, detailed speed profiles were obtained for a light passenger vehicle (a Toyota Kijang) and a medium truck on a 17 km congested road close to Bandung in Java. In total 100,000 speeds and 10,000 fuel readings were measured over 1000 km of test runs. Speed data were compared with predictions of the Indonesian Highway Capacity Manual (IHCM) and observed fuel consumption data were compared with ARFCOM's predictions. It was found that the IHCM model appeared to overestimate the Kijang's speed although a reasonable estimate of the truck's speed was obtained. Total observed fuel consumption was found to be three per cent more for the truck and six per cent more for the Kijang compared with ARFCOM's predictions. The data were analysed in 30 second and four minute intervals and it was found that multiple regression models were able to give good explanations of the IHCM predicted speeds and of the ARFCOM's predicted fuel consumption. However only a very poor explanation of acceleration noise was found from formulae based on the highway volume capacity ratio. In view of the promising results it is proposed that relatively simple multiple regression models be used to predict fuel consumption under congestion. (a)
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