Loose Change

Vehicle trip generation forecasts for new property developments involve far more art than science.

Plenty has been written about the deficiencies of the ITE Trip Generation Manual (from the US, currently in its 8th Edition). However, if used as a starting point (with care taken to note the number of data points underlying each forecast category), and not assumed to be overly precise, the manual does provide at least a useful starting point for developing well-considered forecasts. The RTA’s Guide to Traffic Generating Developments (NSW) provides a similar starting point for Australian developments.

A key deficiency of all vehicle trip generation resources is that they apply to single-use land use developments, when indeed all trends in good planning are pushing for mixed-use development patterns. A key benefit of mixed-use development is the ease of which people can naturally walk between proximate destinations - from the office to a cafe for lunch, from work to the bank and onto the train station. Clustering of destinations means that walking is a logical travel choice, and that even those who drive will park once and then walk to multiple destinations. All of these trips are ‘internal’ to the development and do not impact the existing roadway network ‘external’ to the mixed-use development.

However, attempts to quantify the percentage of all vehicle trips generated by the various individual land uses within a mixed-use development that would remain ‘internal’ to the development have largely been one-off estimates using professional judgement bolstered by perhaps a few data points from research. The variation in these estimates can be considerable, and can have a massive impact on the outcome of the overall trip generation forecast, and therefore on real outcomes, such as roadway and parking capacity required for the development.

The recent release of TRB report Enhancing Internal Trip Capture Estimation for Mixed-Use Developments should therefore be greeted with far more enthusiasm than the report’s long-winded title suggests.

The report was many years in development, and even though based on US data points, the methodology and findings can be highly useful for informing Australian vehicle trip generation forecasting. A spreadsheet model is also provided.

The report Sustainable Australia: Containing Travel in Master Planned Estates is also an excellent resource on this issue.

Facilitating walking trips within mixed-use development is one of the most certain and beneficial ways to manage the growing demand for vehicle travel. Let’s be sure the traffic impact assessment process recognises these impacts in a credible way.