Work Zone Lane Closure on a Two-Lane Rural Highway
A two-lane rural highway (one lane each direction) is scheduled for a pavement rehabilitation project. The contractor proposes a flagging operation with a single lane closed for 1.2 km, alternating one-way traffic. The road carries 1,100 veh/h in the peak direction and 400 veh/h in the opposite direction during the peak hour. The speed limit is 90 km/h. The contractor's plan uses one flagger at each end with a 2-minute cycle (1 minute each direction). The engineer is concerned about queue length and the adequacy of the flagging plan.
1. Evaluate whether the proposed flagging operation is adequate, and calculate the expected queue length.
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The flagging operation alternates one-way traffic with a 2-minute cycle (60 s each direction). The capacity of a single-lane alternating operation is governed by the cycle time and the travel time through the work zone.
Travel time through 1.2 km at 90 km/h (25 m/s) = 1,200/25 = 48 s. With a 60 s green per direction, the effective service time per direction is 60 s, but the vehicle must clear the zone before the opposing direction is released. The capacity per direction is roughly:
Capacity = (green time / cycle time) × saturation flow, adjusted for clearance.
With a 60 s green and 48 s travel time, the platoon can clear if the saturation flow is high enough. But the queue depends on the arrival rate versus the discharge rate.
Discharge rate during green: assume saturation flow of 1,800 veh/h (0.5 veh/s). In 60 s, about 30 vehicles discharge. Arrivals in 120 s at 1,100 veh/h = 1,100/3,600 × 120 = 36.7 vehicles. Since 36.7 > 30, the queue grows each cycle. The queue growth per cycle ≈ 6.7 vehicles. Over the peak hour (30 cycles), the queue would grow unbounded - the operation is inadequate.
Queue length: at 6.7 vehicles per cycle and 30 cycles, the queue would exceed the available storage; even the first few cycles produce a queue of ~7 vehicles (about 50 m).
2. Recommend changes to the traffic control plan to make the operation work, and state the trade-offs.
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Options:
- Reduce the cycle time to increase capacity. A shorter cycle (e.g., 40 s each direction) increases the number of discharge opportunities. But the cycle must still allow the platoon to clear the 48 s travel time. If the travel time is 48 s, a 40 s green is too short - vehicles would still be in the zone when the opposing direction is released. So the minimum green must exceed the travel time (48 s).
- Use a pilot car to lead the platoon, which allows a shorter green because the pilot car controls speed and spacing. This can reduce the effective travel time and allow a shorter cycle.
- Close the road at night or use off-peak flagging to avoid the peak demand.
- Use a temporary signal with vehicle detection instead of manual flagging, which can optimize the cycle.
- Reduce the work zone length to reduce travel time and allow a shorter cycle.
Trade-offs: A shorter cycle reduces queue growth but risks releasing opposing traffic before the platoon clears (safety). A pilot car adds cost and requires an extra vehicle/operator. Night work avoids peak demand but increases cost and worker safety risk. The engineer should require the contractor to demonstrate that the minimum green exceeds the travel time and that the capacity exceeds demand (1,100 veh/h).