In order to generate baseline values for time loss due to congestion actual travel times were compared to simulated free-flow travel times using an average free-flow speed of 40 kilometers per hour (km/h). Time loss was calculated as the difference between free-flow travel time and actual travel time over a four hour period from 6 to 10 am.
The simulator was programmed to optimize the network travel time by shifting vehicle departure times or switching vehicle routes. Five alternative routes were available to each vehicle, and departure times could be shifted by up to 30 minutes.
The following three scenarios were analyzed:
Route Switch Scenario-A
Vehicle routing assignments were optimized to improve total network travel time regardless of individual increases or decreases in travel time.
Route Switch Scenario-B
Vehicle routing assignments were optimized to improve the total network travel time, but each vehicle route was altered only if it reduced its own travel time.
Time Shift Scenario
Vehicle departure times were optimized to improve both total network travel time and individual travel times. Departure times were shifted 15 minutes ahead for each vehicle if the individual travel times improved.
The author indicated that travel times in Scenario-A and Scenario-B were dependent on the fixed order in which the individual routes were switched. This limited the precision of the optimization.
- Switching routes in southwest Tokyo lead to a 40 percent decrease in time loss.
- Shifting the departure time of 15 percent of the vehicles by 20 minutes lead to an 80 percent decrease in time loss in southwest Tokyo.
- Shifting the vehicle departure time by 15 minutes eliminated all congestion on the Kasai JCT Tokyo Metropolitan Expressway.
The evaluation framework in this study indicated that small shifts in departure times could greatly improve traffic conditions.
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