Traffic Engineering: Capacity, LOS, and Signal Timing
The organizing framework
Level of service (LOS) is the backbone of this domain, and every facility type uses a different performance measure. Freeway LOS comes from density (pc/mi/ln); signalized intersection LOS comes from control delay (s/veh); unsignalized intersections and roundabouts use delay as well; weaving segments use density and speed. Base saturation flow is 1,900 pc/h/ln, adjusted downward for lane width, heavy vehicles, grade, parking, and lane utilization. Design hourly volume is typically the 30th-highest hour of the year, with K and D factors splitting it into directional design flow.
Signal timing
Webster's optimum cycle is C = (1.5L + 5)/(1 − Σy), where L is total lost time and y = v/s is the critical flow ratio for each phase. Green splits are proportional to those y values. The change interval is yellow = t + v/(2a + 2gG), with t ≈ 1.0 s, a ≈ 10 ft/s², plus an all-red clearance.
Traps
- Using v/c ratio as if it were LOS; it is only an input.
- Computing PHF as peak-hour volume divided by the peak 15-minute volume (the denominator must be 4 × the peak 15).
- Applying freeway thresholds to arterials, or forgetting pedestrian minimum green when checking splits.
- Ignoring that oversaturated conditions make Webster's cycle meaningless.
Worked example
A count shows 2,000 veh in the peak hour and 560 veh in the busiest 15 minutes. PHF = 2,000 / (4 × 560) = 0.89. If the peak 15-minute flow rate is 560 veh/15 min (2,240 veh/h) and saturation flow is 1,900 pc/h/ln over two lanes, then v/c = 2,240 / 3,800 ≈ 0.59 - a comfortable LOS, so the study should check whether the design hour, not just the count hour, governs.