Potential applications of speed dispersion in defining freeway level of service and air emissions evaluation

Speed dispersion, with respect to its statistical relationships with fundamental traffic flow parameters, is used to measure freeway level of service in the context of ‘no more than x% of vehicles with travel time up to y% greater than the free flow condition.’ It is argued that such a measure not only reflects mobility, reliability, and – potentially – safety, but also avoids the vague descriptions associated with each service level in the current US Highway Capacity Manual. These relationships are then applied to produce speed distribution estimates for the MOtor Vehicle Emissions Simulator (MOVES) model. It is argued that the MOVES model's approach limits the distribution in two speed bins, results in unsupported speed dispersion, and may cause identical distributions under various average speeds. The proposed revised approach based on speed dispersion generates specific spread-out distributions consistent with empirical data. The findings of these two applications bring new concepts to current practice.