The simple capacity model gives some general insights on roadway performance

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Model results
Result 1. Assumptions 1-4 are sufficient to "show" that two lanes have twice the capacity of one lane. In reality, when travel speeds are less than 30 mph, a second lane more than doubles the capacity by enabling faster vehicles to pass slower ones, thus raising the average travel speed (an effect ignored in this simple model).

Result 2. The model including an assumption for follow-up headway predicts the capacity of a single lane road as a function of the travel speed.

Result 3. We can understand the qualitative shape of the curves. The capacity approaches a constant value at high speed, since, for constant hw, the separation distance increases as the speed increases. At low speeds, less than 20 mph, the capacity decreases rapidly because the cars' length occupies significant road space.

Result 4. We can extract a few nominal (hw = 3 sec) cases that are relevant to suburban and urban roadways.

Speed (mph)
Separation Distance (ft)
Capacity (veh/hr)
50
220
1130
35
150
1100
20
88
1030
10
44
910
Result 5. Changes in headway due to driver aggressiveness or adverse conditions cause large changes in roadway capacity (see plot above).

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