Intersection Capacity is more than the volume of vehicles the intersection can accommodate in a given time interval. First, as transportation engineers we must give consideration to pedestrians, mass transit, and other forms of travel. Second, we must design for the time required for an intersection to fill up and then empty given the various speeds of pedestrians, private vehicles, buses, light rail, etc. And third, we must be able to coordinate each intersection with the overall traffic configuration of other intersections, traffic flow patterns, access points, speed limits, etc.
The study of intersection capacity utilization is filled with opportunities for creativity and problem-solving.
Using information from Traffic Engineering – Street Segment Interrupted Flow and based on the traffic counts for the intersection shown (assume up is north):
1. Calculate the East-West Critical Lane Volumes:
- East bound traffic turning left + (West bound traffic + West bound traffic turning right) = 105 + (250 + 130) = 105 + 380 = 485 vph
- West bound traffic turning left + (East bound traffic + East bound traffic turning right) = 70 + (215 + 60) = 70 + 275 = 345 vph
2. Calculate the North-South Critical Lane Volumes:
- North bound traffic turning left + (South bound traffic + South bound traffic turning right) = 75 + (245 + 65) = 75 + 310 = 385 vph
- South bound traffic turning left + (North bound traffic + North bound traffic turning right) = 110 + (225 + 125) = 110 + 350 = 460 vph
3. Calculate the Critical Volume for Phase (Vci):
- Vci = 485 vph (from calculations in steps 1 and 2)
4. Calculate the Total Critical Volume (Vc):
- Vc = sum of Critical Lane Volumes = 380 + 275 + 310 + 350 = 1315 vph
5. Calculate the Saturation Flow Rate (given: headway (h) = 2.3 sec/veh):
- Saturation Flow Rate (s) = 3600/h = 3600/2.3 = 1565 veh/hr
6. Calculate the Cycle Length (C) if the number of phases (N) = 4, the total lost time per phase (tL) = 4 sec, the Peak Hour Factor (PHF) = 1.0, and the desired volume/capacity (v/c) ratio = 0.97:
- C = (N x tL) ÷ [1 – (Vc ÷ (PHF x (v/c) x s))]
- C = (4 x 4) ÷ [1 – (1315 ÷ (1.0 x 0.97 x 1565))] = 120 sec
7. Calculate the Effective Green Time (gi):
- gi = (Vci ÷ Vc) x (C – L), where Vci = Critical Volume for Phase, Vc = Total Critical Volume, and L = Total Lost Time = N x tL
- gi = (485 ÷ 1315) x (120 – (4 x 4)) = 38.4 sec
8. Calculate the capacity of one approach lane:
- c = 3600gi ÷ hC = (3600 x 38.4) ÷ (2.3 x 120) = 500vph
Related Content:
- Traffic Signal Timing Manual
- Standardization of Yellow and All- Red Intervals for Signalized Intersections
- Intersections
- Signalized Intersections
- Intersection Capacity Utilization
- Signalized Intersections: Informational Guide
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