Freeway Capacity Estimation Method for Planning Applications

Abstract

The capacity of a freeway segment is a critical factor for the planning, design, and analysis of freeway facilities. The Highway Capacity Manual (HCM) is considered to be one of the authoritative sources on capacity values for a variety of roadway types in the United States, particularly for planning purposes. For basic freeway segments, a single set of capacity values are provided as a function of free-flow speed. Although these values are considered to be reasonably representative values for freeways located throughout the United States, the HCM does not provide any guidance on how its recommended values can be adjusted to reflect significant differences in capacity due to local conditions, nor how to directly measure or estimate capacity values. With the recent development of a statewide freeway traffic data archive in Florida, the Florida Department of Transportation (FDOT) desired to use these data to determine if the freeway capacity values in the HCM were appropriate for Florida freeway level of service analyses for planning and preliminary engineering applications. This study evaluated two previously published methods for estimating freeway capacity, identified their advantages and disadvantages for use in planning applications, and ultimately recommended a method for use by FDOT. One of the chosen methods determines capacity from a mathematical function generated from speed-flow data points and the other chosen method uses the concept of breakdown probability distribution to determine capacity. Neither of these methods proved desirable for planning applications due to computational burden; thus, an alternate, simplified approach was developed that used a simple averaging method of highest flow rates to determine capacity.