Many recent green vehicle routing papers have presented mathematical models designed to minimize fuel consumption and the environmentally damaging carbon emissions in the routing decisions related to route choice, the load on the vehicle, and the speed. A popular model for computing such impact is the Comprehensive Modal Emissions Model (CMEM) that can compute fuel consumption and carbon emissions for a vehicle with a given speed and load. However, the CMEM requires that input parameters are specified for every second of a haul. To avoid this, computations are used in which the vehicle is assumed to travel at a fixed speed. This paper investigates the extent to which such fixed speed computations in the CMEM are suitable for actual driving conditions with fluctuating speeds. In our numerical experiments, we find that the CMEM results under fixed speeds are sometimes less than half of those computed under realistic driving conditions, depending on the type of conditions. This indicates that we cannot take for granted that fixed speed computations are sufficiently accurate to be used in green routing and validation of these computations is necessary.

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