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8 TABLE 1 Passenger car equivalent (PCE) capacities for freeways and highways Free-Flow Speed PCE Capacity (passenger cars per hour per lane) Freeways Multilane Hwys Two-Lane Hwys 75 mph (112 km/h) 2400 70 mph (104 km/h) 2350 65 mph (96 km/h) 2300 2200 1700 60 mph (88 km/h) 2250 2100 1700 55 mph (80 km/h) 2000 1700 50 mph (70 km/h) 1900 1700 Where: DL = segment delay between signals (equals zero if no USER'S GUIDE signals) (hours), c = capacity (vph), N = number of signals on the segment (equals one if no s0 = ideal saturation flow rate = 1,900 vehicles per hour signals), of green per lane, T = expected duration of the demand (length of analysis N = number of lanes, period) (hours), fw = lane-width adjustment factor, x = segment demand/capacity ratio, fhv = heavy-vehicle adjustment factor, L = segment length (miles), and Fg = grade adjustment factor, J = calibration parameter. fp = on-street parking crossing adjustment factor, fbb = local bus adjustment factor, The segment traversal time at free-flow conditions is com- fa = central business district adjustment factor, puted from the free-flow speed: fLU = lane use adjustment factor, fLT = left-turn adjustment factor, R0 = L S Equation 6 fRT = right-turn adjustment factor, 0 fLpb = pedestrian/bicycle blockage of left-turn factor, fRpb = pedestrian/bicycle blockage of right-turn factor, Where: PHF = peak-hour factor, and R0 = free-flow traversal time (hours), g/C = ratio of effective green time per cycle. L = length (miles), and S0 = the segment free-flow speed (mph). See the HCM for appropriate values for the adjustment factors. The computation of the signal delay terms (D0, DL) is explained in the following section. The number of signals (N) on the facility segment excludes 3.3 HCM/AKCELIK SPEED-FLOW EQUATION the signal at the start of the street segment (if present), because this signal should already have been counted in the upstream The mean speed for each segment during the peak period segment. (Streets are often split into segments (links) starting is estimated using the following equations taken from the and ending at signalized intersections. The counting conven- 2000 HCM. The mean vehicle speed for the link is computed tion suggested here avoids double-counting of the signals by dividing the link length by the link traversal time. The link located at the start and end points of each segment.) traversal time (R) is computed according to the following When there are no signals on the facility, N is still set modified Akcelik equation from the HCM: equal to one. This is because N is really the number of "delay- causing elements" on the facility. Each delay-causing ele- R = R0 + D0 + DL + 0.25 N ment on the facility adds to the overall segment delay when 16 J L2 x demand starts to approach and/or exceed capacity at that ele- T ( x - 1) + ( x - 1)2 + N 2T 2 Equation 5 ment or point. Because demand in excess of capacity must wait its turn to enter the facility segment, there is always at least one "delay-causing element" (the segment itself) on a Where: facility even when there are no signals. The more signals R = segment traversal time (hours), there are on a facility, the more points there are where traffic R0 = segment traversal time at free-flow speed (hours), is delayed along the way. This means that a bottleneck sec- D0 = zero-flow control delay at signals (equals zero if no tion of the facility should be coded as a single link and not signals) (hours), arbitrarily split into sublinks. The HCM/Akcelik equation
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9 (and the standard BPR equation as well) treats each link as a ( Rc - R0 - D0 - DL )2 potential delay-causing bottleneck on the network. Splitting J= Equation 7 L2 one real-world bottleneck into three hypothetical links, each with the same demand, would triple the estimated delay at the Where: bottleneck. The duration of demand (T) is set equal to the length of the J = calibration parameter, analysis period. Rc = link traversal time when demand equals capacity The segment demand/capacity ratio (x) is the ratio of the (hours), total demand for the analysis period divided by the total R0 = free-flow speed traversal time (hours), capacity for the period. D0 = zero-flow control delay (hours), and The calibration parameter J is selected so that the traver- DL = segment delay (hours). sal time equation will predict the mean speed of traffic (aver- aged over the length L of the link) when demand is equal to The values for J, shown in Tables 2 and 3, reproduce the capacity. It is computed according to the following equation: mean segment speeds at capacity predicted by the analysis USER'S GUIDE TABLE 2 Recommended calibration parameters J for freeways and highways SI Units Facility Type Signals Per Km Free-Flow Speed (km/h) Speed at Capacity (km/h) J Freeway n/a 120.0 85.7 1.11E-05 Freeway n/a 110.0 83.9 8.00E-06 Freeway n/a 100.0 82.1 4.75E-06 Freeway n/a 90.0 80.4 1.76E-06 Multilane Hwy n/a 100.0 88.0 1.86E-06 Multilane Hwy n/a 90.0 80.8 1.60E-06 Multilane Hwy n/a 80.0 74.1 9.91E-07 Multilane Hwy n/a 70.0 67.9 1.95E-07 Two-Lane Hwy n/a 110.0 70.0 2.70E-05 Two-Lane Hwy n/a 100.0 60.0 4.44E-05 Two-Lane Hwy n/a 90.0 50.0 7.90E-05 Two-Lane Hwy n/a 80.0 40.0 1.56E-04 Two-Lane Hwy n/a 70.0 30.0 3.63E-04 Customary Units Facility Type Signals Per Mile Free-Flow Speed (mph) Speed at Capacity (mph) J Freeway n/a 75.0 53.3 2.947E-05 Freeway n/a 70.0 53.3 2.003E-05 Freeway n/a 65.0 52.2 1.423E-05 Freeway n/a 60.0 51.1 8.426E-06 Freeway n/a 55.0 50.0 3.306E-06 Multilane Hwy n/a 60.0 55.0 2.296E-06 Multilane Hwy n/a 55.0 51.2 1.821E-06 Multilane Hwy n/a 50.0 47.5 1.108E-06 Multilane Hwy n/a 45.0 42.2 2.174E-06 Two-Lane Hwy n/a 65.0 40.2 9.043E-05 Two-Lane Hwy n/a 60.0 35.2 0.0001385 Two-Lane Hwy n/a 55.0 30.2 0.0002239 Two-Lane Hwy n/a 50.0 25.2 0.0003893 Two-Lane Hwy n/a 45.0 20.2 0.0007484