Effect of grade on the relationship between flow and occupancy on freeways

In this paper, 30-sec data for the median lane of a freeway section with significant grades are used to explore how the uncongested flow-occupancy function is affected by grade, as well as to examine the implication of this effect for incident detection logic. Significant grades cause differences in speed at affected locations, resulting in differences in occupancy for a given flow. The size of the difference depends not only on the grades at each location but also on the vertical profile of the upstream sections and on the prevailing flow. Distinct flow-occupancy functions are required for each affected location, and because the effect of the grade might vary with environmental and traffic operating conditions, it appears desirable to continuously update these functions on line in traffic management systems.