Guide for Highway Capacity and Operations Analysis of Active Transportation and Demand Management Strategies
Appendix D: Speed/Capacity for Work Zones
The 2010 Highway Capacity Manual and SHRP 2-L08 provide recommended capacity adjustments for freeway work zones. More recent and comprehensive information on the traffic analysis of work zones can be found in Traffic Analysis Toolbox Volume XII (Zhang, Morallos, Jeannotte, & Strasser, 2012). The National Cooperative Highway Research Project (NCHRP) 3-107 will produce additional guidance on the capacity and traffic speeds in work zones in late 2014.
Work zones include short-term work zone lane closures due to maintenance and long-term lane closures due to construction. According to the Manual on Uniform Traffic Control Devices (MUTCD), construction duration for long-term work zone is more than three days and could last several weeks, months, or even years, depending on the nature of works. Short-term work zone duration is more than an hour and within a single daylight period (MUTCD, 2009). Long-term construction zones generally use portable concrete barriers, while short-term work zones use standard channelizing devices.
Chapter 10 of the 2010 Highway Capacity Manual (HCM) summarized the lane closures and ranges of capacity during construction (HCM 2010). Exhibit 10-14 of the 2010 HCM provides work zone capacities in terms of vehicles per hour per lane according to the original number of lanes (before work zone) and the number of lanes open when the work zone is in place (Table 42).
Source: Default values and ranges from Exhibit 10-14 2010 HCM; values shown are vehicles per hour per lane unless otherwise noted.
Note: Pc/hr/ln (passenger cars per hour per lane) equivalent computed assuming level terrain, 5% heavy vehicles, and 0.90 PHF.
The vehicle per hour per lane capacities (veh/hr/ln) in Exhibit 10-14 of the HCM were converted to passenger car equivalents for the purpose of computing capacity adjustment factors for work zones. The capacity adjustment factors for a 65 mph free-flow speed freeway are computed assuming that the values in Exhibit 10-14 of the HCM apply to 65 mph free-flow speed freeway with a base (dry weather, nonwork zone capacity of 2,300 pc/hr/ln. The same capacity adjustment factors computed for a 65 mph free-flow speed freeway are assumed to apply to freeways with higher and lower free-flow speeds. In other words, the effect of the work zone on capacity is assumed to be proportional to the base capacity. The resulting capacity adjustment factors applicable to all freeways, regardless of free-flow speed are shown in Table 43. We have extrapolated Exhibit 10-14 of the 2010 HCM to freeway work zones with 5 moving lanes.
|Number of Lanes Open in Work Zone||Work Zone Capacity Adjustment Factor|
Note: Work Zone Capacity = (Base Capacity) * (Capacity Adjustment Factor).
The SHRP 2-L08 recommends the capacity adjustment factors shown in Table 44. These factors are computed for an assumed 55 mph free-flow speed within the work zone and a base capacity of 2,250 pc/hr/ln.
|Directional Lanes (One Direction, Before Work Zone)||1 Lane Closed||2 Lanes Closed||3 Lanes Closed|
Note: N/A = Not Applicable, Source: Exhibit 36-17, Draft HCM Chapter, SHRP 2-L08.
Neither source, HCM 2010 or SHRP 2-L08, identify free-flow speed adjustment factors to apply in work zones. The analyst may consider reductions to account for the lower posted speed limits in freeway work zones, rubbernecking due to driver distractions (and entering and exiting construction vehicles) associated with work zones.previous | next