Signalized Intersection Timing and Pedestrian Conflict
A four-leg signalized intersection has an 11-ft crosswalk on the east leg and a 24-ft travel-way width. The cycle length is 90 s. The east-west through movement has a saturation flow of 1,800 veh/h/lane and a demand of 1,400 veh/h in two lanes. The north-south left-turn phase is protected and has a demand of 250 veh/h with a saturation flow of 1,700 veh/h. The intersection has a pedestrian signal with a 7-s flashing DON'T WALK and a 3-s buffer. The engineer proposes to reduce the cycle to 60 s to reduce delay. Field observation shows pedestrians frequently still in the crosswalk when the parallel green begins.
1. Check whether the pedestrian clearance interval is adequate for the 24-ft crossing.
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Pedestrian clearance time = crossing distance / walking speed + buffer. Using the standard 3.5 ft/s walking speed and a 24-ft crossing: 24 / 3.5 = 6.9 s, plus a 3-s buffer = 9.9 s total clearance. The provided flashing DON'T WALK is 7 s plus a 3-s buffer = 10 s, which is marginally adequate (10 s ≥ 9.9 s). However, the 11-ft crosswalk width is the width of the crosswalk, not the crossing distance - the crossing distance is the 24-ft travel-way. The observed pedestrians still in the crosswalk at green onset suggest the walking speed assumption is too high for this population (e.g., near a school or senior center), or the 7-s flash starts late. A defensible fix is to use a slower walking speed (3.0 ft/s: 24/3.0 = 8.0 s + 3 = 11 s) and lengthen the flash to 8–9 s.
2. Evaluate the effect of reducing the cycle from 90 s to 60 s on the pedestrian clearance and on intersection operation.
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The pedestrian clearance interval is a fixed minimum driven by geometry and walking speed; it does not shrink with the cycle. Shortening the cycle to 60 s does not reduce the required 10–11 s of clearance - it consumes a larger fraction of the cycle. At 90 s, 10 s of clearance is 11% of the cycle; at 60 s it is 17%, leaving less green time for vehicles and increasing the likelihood of pedestrian/vehicle conflict. On the vehicle side, a shorter cycle generally reduces uniform delay but increases the number of lost-time intervals per hour, which raises delay when volumes are high. With the east-west demand at 1,400/3,600 = 0.39 of saturation in two lanes, the intersection is not heavily loaded, so the shorter cycle may help vehicles - but only if the pedestrian clearance is preserved and the lost time is accounted for. The proposal is not automatically beneficial and must be checked against the pedestrian timing constraint first.
3. Compute the approximate degree of saturation for the east-west through movement and comment.
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Capacity of two lanes = 2 × 1,800 = 3,600 veh/h. Demand = 1,400 veh/h. Degree of saturation = 1,400 / 3,600 = 0.39. That is a lightly loaded movement; the through movement is not the constraint. The north-south protected left has capacity 1,700 veh/h and demand 250 veh/h → v/c = 0.15, also light. The real constraint at this intersection is therefore likely the pedestrian timing and lost time, not vehicular capacity. This supports keeping a longer cycle (or at least not shortening it below the pedestrian requirement), and it means the delay reduction from a 60-s cycle will be modest. The engineer should focus on the pedestrian clearance and the phase sequence rather than the cycle length.