Work Zone Free Flow Speed Calculation Issues in HCM 2016

Capacity and free flow speed (FFS) are the two most important traffic characteristics that determine traffic flow conditions in work zones. Work zone capacity and FFS are also important because they directly affect queue length and delay. Delay and queue length are commonly used in performance evaluation, design, and planning of freeway work zones. Number of queued vehicles influences queue length and delay, heavily relies on work zone capacity and speed of vehicles in the work zone. Highway Capacity Manual 2016 (HCM 2016) developed new procedures to calculate work zone capacity and FFS (1). Work zone capacity and FFS are independently calculated to create a speed-flow curve, which represents the traffic operation in the work zone. The HCM 2016 FFS calculations provide acceptable results for some work zones, but it computes counter-intuitive FFS for some realistic work zone conditions. The first counter-intuitive result is when the work zone capacity becomes higher than the capacity of the corresponding basic freeway section (non-work zone). The second is when the estimated work zone FFS becoming higher than the FFS for non-work zone basic freeway section. The last one is when work zone FFS are not consistent with posted speed limit in the work zone and show an opposite trend. It is sensible to expect that work zone capacity to be lower than the capacity of upstream non-work zone freeway section; work zone FFS should be lower than the FFS of a comparable basic freeway section; and work zone FFS should be consistent with work zone speed limit. These issues are examined in this paper, and 5 modified FFS equations are suggested to provide intuitive and acceptable FFS for work zones.