Abstract
Speed variance is an important, but overlooked, parameter in traffic studies. This paper examines the use of statistical relationships between speed variance and the fundamental parameters-density, average speed, and flow-to support applications of speed variance via those fundamental parameters to two important aspects of traffic operations. First, it is proposed that speed variance be used to measure freeway level of service from A through E in the context of “no more x% of the vehicles with travel time up to y% greater than the free flow condition.” It is argued that such a measure not only reflects mobility (y%), reliability (x%), and potentially safety, but also avoids the vague descriptions associated with each service level in the current HCM. Second, the relationships are applied to estimate speed distribution for the MOVES mobile source air emission model. A revised approach is developed and compared to that in the current model. It is argued that the MOVES model’s approach limits the distribution in two speed bins, results in unsupported speed variance, and may cause identical distributions under various average speeds. The proposed revised approach based on speed variance generates specific spread-out distributions consistent with empirical data. The findings of these two applications bring new concepts to the current practice. Suggestions are made to fix the deficiencies of the existing and proposed approaches in the applications.