Civil Engineering Reference
In-Depth Information
fast. If it cannot find a way to balance car numbers with road capacity, the
model can be run through an additional iteration, in which it loads cars
along different routes (as drivers themselves would try alternatives, based
on experience) to pick less congested routes.
In our model of Square City, we continue our runs of the model until
the tenth iteration. We then consult the model to tell us the speeds on
specific road segments. We are particularly interested in the existing bridge,
known as Old Bridge, which is Square City's chokepoint, where the speed
limit is 45 mph. Figure 8.3 shows minute-by-minute average speeds on the
bridge during rush hour. We see that traffic on the bridge starts off at the
free-flow rate of 45 mph. As traffic builds up, average speed declines, though
at different rates for each of the two traffic directions. After 25 minutes,
the traffic is creeping at 8 mph, and remains at that speed until the end
of the hour.
The software through which we run our model can also generate maps
of traffic location in Square City. As we would expect, the maps (not shown)
of conditions during rush hour show traffic backing up at approaches to
the bridge.
SQUARE CITY UNDER FOUR SCENARIOS
We have just described the results under the current population and the
single existing bridge. Let's call this scenario 1, the base scenario. Being
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Peak Hour
Figure 8.3. Average traffic speed (in each direction) on Old Bridge during peak
travel hour.
