Signal or Flyover: Justifying a Two-Phase Signal at a Four-Arm Intersection
A four-arm at-grade intersection on a state highway in a tier-2 city carries peak-hour flows of 1,150 PCU/h from the north, 950 from the south, 780 from the east and 640 from the west. Each approach has two lanes and the saturation flow is 1,800 PCU/h per lane. The intersection is currently uncontrolled; there were six reported accidents in two years, two of them fatal and involving two-wheelers, and pedestrians cross the state-highway arm in large numbers during school hours. The municipal proposal is a two-phase signal. The PWD counter-proposal is a grade-separated flyover on the north–south arm at about 25 times the signal cost. Traffic is growing at about 5% per year, and long-term plans include widening all approaches to three lanes within five years.
1. Design the two-phase signal by Webster's method and check whether it has adequate capacity at the present flows.
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Critical flow ratios (each approach: 2 lanes × 1,800 = 3,600 PCU/h):
- NS phase: y = max(1,150, 950)/3,600 = 1,150/3,600 = 0.319
- EW phase: y = max(780, 640)/3,600 = 780/3,600 = 0.217
- Y = 0.319 + 0.217 = 0.536
Lost time: 2 phases × (4 s start-up + 2 s amber) = 12 s
Optimum cycle: Co = (1.5L + 5)/(1 − Y) = (18 + 5)/(1 − 0.536) = 23/0.464 = 49.6 → adopt 50 s
Green splits: total effective green = 50 − 12 = 38 s
- NS = 38 × 0.319/0.536 = 22.6 s ≈ 23 s
- EW = 38 × 0.217/0.536 = 15.4 s ≈ 15 s
Capacity check:
- NS capacity = 3,600 × 23/50 = 1,656 PCU/h against 1,150 → v/c = 0.69
- EW capacity = 3,600 × 15/50 = 1,080 PCU/h against 780 → v/c = 0.72
Both degrees of saturation are below the practical limit of 0.85–0.90, so a two-phase signal handles the present demand with reserve. Note that adding a pedestrian phase (say 8 s) raises L to 20 s: Co = (30 + 5)/0.464 = 75 s, g_NS = (75 − 20) × 0.319/0.536 = 32.7 s and NS capacity = 3,600 × 32.7/75 = 1,570 PCU/h, i.e. v/c = 0.73 - still adequate, so the pedestrian phase is affordable now.
2. Will the signal still work in year 5? What is the effect of the planned three-lane widening?
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Year 5 flows at 5% growth (factor 1.05⁵ = 1.276):
- NS = 1,150 × 1.276 = 1,468 PCU/h
- EW = 780 × 1.276 = 995 PCU/h
With two lanes: y_NS = 1,468/3,600 = 0.408; y_EW = 995/3,600 = 0.276; Y = 0.684. Co = 23/(1 − 0.684) = 72.8 → adopt 75 s. Effective green = 63 s; g_NS = 63 × 0.408/0.684 = 37.6 s; NS capacity = 3,600 × 37.6/75 = 1,805 PCU/h → v/c = 0.81.
So the two-lane signal would still work at year 5, but only just - 0.81 with no pedestrian phase, and above 0.85 once one is added. It would be at the edge, with long queues and cycle failure on any incident day.
With three lanes (saturation 5,400 PCU/h): y_NS = 1,468/5,400 = 0.272; y_EW = 995/5,400 = 0.184; Y = 0.456; Co = 23/0.544 = 42 s. Capacity at a 60 s cycle with 48 s effective green: NS = 5,400 × 48 × 0.272/0.456/60 ≈ 2,580 PCU/h against 1,468 → v/c ≈ 0.57.
The widening alone restores a comfortable margin for a further 8–10 years. That is the decisive finding for the flyover argument: the capacity problem is solvable at-grade for well under a tenth of the flyover cost.
3. Recommend a scheme, addressing the accident record and pedestrians as well as capacity.
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Recommendation: signal now, widen within five years, defer the flyover.
Reasons:
- Capacity is not the case for a flyover. At present flows v/c is 0.69–0.72; after widening it falls to about 0.57. A 25× cost premium is not justified by a capacity shortfall that does not exist.
- Safety is a separate and immediate warrant. Six accidents in two years with two fatalities, on an uncontrolled intersection with heavy pedestrian crossing, justifies signalisation on its own. IRC warrants treat accident history, pedestrian volume and the major/minor road volume balance as independent grounds, and all three point the same way here. Waiting five years for a flyover would keep the conflict point live.
- A flyover does not solve the intersection. An NS flyover removes the north–south through movement but leaves the turning movements, the east–west crossing and the pedestrian conflict at grade, often at higher speed and therefore higher severity. It can worsen the safety position for pedestrians unless a subway is added.
Scheme to sanction: two-phase signal with a protected pedestrian phase and countdown display; channelising islands with a free-flow left-turn slip for the lighter east and west movements; median refuge and a footpath/guard rail on the school side; enforcement of parking and encroachment. Reserve the flyover option only if, after widening, the measured v/c exceeds 0.90 or queues block the upstream junction - and use the interim to build a pedestrian subway at the school crossing.