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Bypasses of regional scope serve to relieve traffic congestion on a parallel road and to give drivers not needing to stop locally a faster alternative route to their destination. Such bypasses have more than a few intersecting streets and provide access to substantial swaths of land that are not accessible to the original route.
Bypasses of a regional scope require a depth of analysis provided by a travel forecasting model. The travel forecasting model may have already been developed for long-range planning, or the travel forecasting model could be developed specifically for this bypass. If applicable, a custom model could use either the windowing technique (Windowing with OD table estimation from traffic counts) or the subarea focusing technique (Subarea focusing). The decision to use a custom model instead of an existing model depends upon the adequacy of the existing model. The existing model should be reviewed for:
- Size of zones, for sufficient spatial precision;
- Analysis time period, for sufficient temporal precision;
- Delay relationships, for reasonable estimates of diversion due to congestion and traffic controls;
- Validation accuracy on the original route and on major crossing arterials;
- Age of input data.
In either case, the outputs of the selected travel forecasting model may be refined by comparison to screenline counts or to forecasts done by a statistical (e.g., time series) method.