An Interpretation of the Parameters in the Simple Average Travel Speed Model of Fuel Consumption

Abstract

The simple average travel speed model of fuel consumption is widely used but the interpretation of its parameters and its areas of use have not been well understood. By deriving aggregate (but detailed) fuel consumption functions from a new instantaneous fuel consumption function, the coefficient of the average speed term can be shown to equal the idle fuel rate and the constant term can be shown to be related to the fuel required to provide tractive force to the vehicle. Thus, the constant term accounts for drag, inertia and grade components of fuel consumption and is influenced by vehicle parameters such as mass, energy efficiency, rolling resistance and aerodynamic drag as well as the driving environment. A method is given for adjusting the constant term for different vehicle parameters and driving environments. The effect of varying the vehicle's mass is discussed in detail. The average travel speed model should only be used for average speeds of less than 50 km/h since the model does not adequately reflect the increase in aerodynamic drag at high speeds. More detailed models should therefore be used for average travel speeds over 50 km/h and references are given for these models. The model is suitable for assessing the impacts on fuel consumption of transport management schemes but not of detailed traffic design schemes.