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).
|