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7 CHAPTER 3 THE HCM ASSIGNMENT MODULE The purpose of the HCM Assignment Module is to improve 3.2 CAPACITIES current methods for estimating the travel delay effects of traf- fic congestion. The approach taken was to replace the con- Highway link capacities are estimated using the proce- USER'S GUIDE ventional Bureau of Public Roads (BPR) equation method dures contained in the 2000 HCM. The following subsec- still used in many travel demand models with more up-to- tions summarize the information contained in Chapter 30 of date traffic operations research results contained in the 2000 the HCM. HCM. The module substitutes the following HCM-based information into the SUE traffic assignment step of the 3.2.1 Freeways, Multilane Highways, travel demand model process: and Two-Lane Highways Free-flow speeds by facility type and area type; The following equation is used to compute the capacity of Link capacities by facility type, area type, and other a freeway or highway link at its critical point. The critical characteristics of facility; and point is the point on the link with the lowest throughput HCM-based Akcelik set of speed-flow equations. capacity. c = Q N Fhv Fp Fg PHF Equation 3 3.1 FREE-FLOW SPEEDS Where: The free-flow speed is the mean speed of traffic when c = capacity (vph), demand is so low that changes in demand do not affect the Q = the passenger car equivalent (p.c.e.) capacity per mean speed of traffic on the segment. For freeways and multi- hour per lane, lane highways, free flow is the mean speed observed when N = number of through lanes (ignore auxiliary and "exit volumes are under 1,300 vehicles per hour per lane. For sig- only" lanes), nalized streets, the free-flow speed is the maximum mean Fhv = heavy-vehicle adjustment factor, speed of traffic obtained at any point between signalized Fp = driver population adjustment factor, intersections for low-volume conditions. Fg = grade adjustment factor, and The mean speed is computed as the sum of the travel times PHF = peak-hour factor. to traverse the length of the segment, divided into the length of the segment times the number of vehicles in the sample. Table 1 provides the HCM-recommended passenger car The following linear equations from NCHRP Report 387 can equivalent capacities per lane (Q). See the HCM for appro- be used to estimate free-flow speed based on the posted speed priate values for the adjustment factors. limit for arterials, freeways, and highways. For posted speed limits of 50 mph or greater, 3.2.2 Arterials FFS = 0.88 PSL + 14 Equation 1 The capacity of an arterial is determined by examining the For posted speed limits of less than 50 mph, through movement capacity at each signal-controlled inter- section on the arterial link. The intersection with the lowest through capacity determines the overall capacity of the arte- FFS = 0.79 PSL + 12 Equation 2 rial link. The following equation is used to compute the one- direction through capacity at each signal. Where: FFS = free-flow speed (mph) and c = S0 N fw fhv Fg fp fbb fa fLU Equation 4 PSL = posted speed limit (mph). fLT fRT FLpb fRpb PHF g/C