Module 7 : Traffic Signal Design

Lecture 37 : Capacity and Los Analysis of a Signalized I/S

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	Determining saturation flow rate A saturation flow rate for each lane group is computed according to above equation. The saturation flow rate is the flow in vehicles per hour that can be accommodated by the lane group assuming that the green phase were displayed 100 percent of the time (i.e., g/C = 1.0). $$S= S_O \times N f_{w} f_{HV} f_{g} f_p f_{bb} f_a f_{LU} f_{LT} f_{RT} f_{Lpb} f_{Rpb}$$ (1) where, S = saturation flow rate for subject lane group, expressed as a total for all lanes in lane group (veh/h); $S_{O}$ = base saturation flow rate per lane (pc/h/ln); N = number of lanes in lane group; $f_{w}$ = adjustment factor for lane width; $f_{HV}$ = adjustment factor for heavy vehicles in traffic stream; $f_{g}$ = adjustment factor for approach grade; $f_{p}$ = adjustment factor for existence of a parking lane and parking activity adjacent to lane group; $f_{bb}$ = adjustment factor for blocking effect of local buses that stop within intersection area; $f_{a}$ = adjustment factor for area type; $f_{LU}$ = adjustment factor for lane utilization; $f_{LT}$ = adjustment factor for left turns in lane group; $f_{RT}$ = adjustment factor for right turns in lane group; $f_{Lpb}$ = pedestrian adjustment factor for left-turn movements; and $f_{Rpb}$ = pedestrian-bicycle adjustment factor for right-turn movements. Base saturation flow rate : For the analysis of saturation flow rate, a fixed volume is taken as a base called base saturation flow rate, usually 1,900 passenger cars per hour per lane (pc/h/ln). This value is adjusted for a variety of conditions. The adjustment factors are given below. Adjustment for lane width: The lane width adjustment factor fw accounts for the negative impact of narrow lanes on saturation flow rate and allows for an increased flow rate on wide lanes. The lane width factor can be calculated for lane width greater than 4.8m. The use of two narrow lanes will always result in higher saturation capacity than one single wide lane. $$f_w = 1+ (w-3.6)/9$$ (2) where, w = width of lane Adjustment for Heavy Vehicles and Grade : passenger cars are affected by approach grades, as are heavy vehicles. The heavy-vehicle factor accounts for the additional space occupied by these vehicles and for the difference in operating capabilities of heavy vehicles compared with passenger cars. The passenger-car equivalent (ET) used for each heavy vehicle is 2.0 passenger-car units and is reflected in the formula. The grade factor accounts for the effect of grades on the operation of all vehicles. $$f_{HV} = 100/ [100 + \%HV (E_T - 1) ]$$ (3) $$f_g = 1- \%G / 200$$ (4) where, % HV = % heavy vehicles for lane group volume, $E_T$ = 2.0, % G = % grade on a lane group approach Adjustment for Parking Parking maneuver assumed to block traffic for 18 s. Use practical limit of 180 maneuvers/h. The parking adjustment factor, $f_p$, accounts for the frictional effect of a parking lane on flow in an adjacent lane group as well as for the occasional blocking of an adjacent lane by vehicles moving into and out of parking spaces. Each maneuver (either in or out) is assumed to block traffic in the lane next to the parking maneuver for an average of 18 s. $$f_P = [ N - 0.1 - (18N_m/3600 ) ] / N$$ (5) where, $N_m$ = number of parking maneuvers/h, N = no. of lanes Adjustment for Bus Blockage The bus blockage adjustment factor, $f_{bb}$, accounts for the impacts of local transit buses that stop to discharge or pick up passengers at a near-side or far-side bus stop within 75 m of the stop line (u/s or d/s). If more than 250 buses per hour exist, a practical limit of 250 should be used. The adjustment factor can be written as, $$f_{bb} = [N - (14.4 N_B/3600)] /N$$ (6) where, $N_B$ = no. of buses stopping per hour Adjustment for Area Type The area type adjustment factor, fa, accounts for the relative inefficiency of intersections in business districts in comparison with those in other locations. Application of this adjustment factor is typically appropriate in areas that exhibit central business district (CBD) characteristics. It can be represented as, $f_a$ = 0.9 in CBD (central business district) and = 1.0 in all others Adjustment for Lane Utilization The lane utilization adjustment factor, fLU, accounts for the unequal distribution of traffic among the lanes in a lane group with more than one lane. The factor provides an adjustment to the base saturation flow rate. The adjustment factor is based on the flow in the lane with the highest volume and is calculated by Equation 10. $$f_{LU} = V_g / (V_{g1}N )$$ (7) where, $V_g$ = unadjusted demand flow rate for lane group (veh/ h), $V_{g1}$ = unadjusted demand flow rate on single lane with highest volume in the lane group and N = no. of lanes in the group.