Issues With Work Zone Free Flow Speed Computation in 6th Edition HCM and Proposed Improvements

Free flow speed (FFS) and capacity of work zones (WZ) directly affect queue length and delay in the WZ. The 6th Edition of Highway Capacity Manual introduced a new model to estimate WZ FFS and virtually an independent model to compute its capacity. Using the FFS and capacity a speed-flow curve is created to represent traffic operation in the WZ. While the new FFS computes reasonable results for some work zones, for many real-world WZ conditions it yields counterintuitive results. The HCM yields WZ FFS values that are higher for the lower WZ speed limits and lower for the higher WZ speed limits; and this is counterintuitive. The main objective of this paper is to alert the users about this problem and propose changes to the HCM WZ FFS model to yield acceptable results. These issues are analyzed and suggested an improved FFS to provide intuitive and acceptable WZ FFS.