Smart Growth Trip Generation Study in California

Residential and office buildings in smart growth areas generate less vehicle traffic than conventional development

What’s the Problem?

The California Environmental Quality Act and other state, federal, and local laws require the identification, analysis, and mitigation of transportation-related impacts of proposed land development projects. One of the first steps in preparing a transportation impact analysis (TIA) is to estimate the number of trips by automobiles, trucks, and other modes of travel that may result from a proposed land development project — a process commonly referred to as “trip generation.”

In most cases, practitioners use vehicle trip generation rates published by the Institute of Transportation Engineers, a national professional organization, or other rates established or accepted by local agencies requesting the TIA. These are derived from data that are almost all from suburban sites in single land use areas and with virtually all trips to and from the sites made by motor vehicle.

However, in recent years, increased development has occurred in areas served by transit and bicycle facilities. Developers have become more sensitized to making developments walkable, and there have been more mixed- and multi-use developments where complementary land uses are mixed closely together, facilitating walk and bicycle travel for some trips. This new (smart growth) style of development generates some trips by non-vehicle modes. Hence, to be credible and accurate, TIAs need to reflect the multimodal trip generation associated with such developments.

Caltrans initiated the Smart Growth Trip Generation (SGTG) project to determine the difference in vehicular trip generation between the suburban-oriented ITE rates and those determined from surveys of California smart growth developments.

How Was the Study Done?

TTI was contracted to produce a validated and improved estimation method and a user-friendly tool to more accurately estimate trip generation for use in determining proper transportation improvements for smart growth developments in California and beyond. To improve the accuracy of the trip generation estimation model previously developed, this project collected trip generation data at approximately 30 smart growth sites and combined it with data already in the Caltrans database.

The project’s four specific objectives were:

1. Achieve significantly improved model accuracy.
2. Increase land use sample sizes to improve estimation accuracy.
3. Develop a model in which the:
   1. Independent variables are widely accepted by TIA preparers and reviewers;
   2. Model is fully transparent and comprehensible to the average user; and,
   3. Future updates can be easily made when additional data become available.
4. Provide effective user training with work facilitated by user-friendly, transparent tools.

Study Findings

Weekday AM and PM peak period multimodal trip generation surveys confirmed that developments of several land uses surveyed indeed do generate fewer vehicle trips than similar developments in single use suburban areas. Depending on specific site and context characteristics, some trips from smart growth sites are made by transit, bicycle and walking. Non-vehicle trips range from a few percent to almost half of all site-generated person trips.

TTI’s approach — with a larger, more consistent database — produced improved results. On average, compared to ITE suburban rates, the surveyed sites generated 44 percent fewer peak hour vehicle trips for apartments and 49 percent less for office buildings. Regression equations using two independent variables — development units and nearby intersection density — provide good, direct estimates of AM and PM peak hour vehicle trip generation for such developments.

The earlier AM and PM models resulted in R² values close to 0.30. TTI’s models had R² values of 0.79 (AM) and 0.85 (PM) for apartment models and 0.73 (AM) and 0.66 (PM) for the office models. The apartment values were similar to those for ITE methods for the same land use (0.83 AM and 0.89 PM). Office values were less than the corresponding ITE values (0.83 AM and 0.82 PM), not surprising for a smaller database.

A spreadsheet estimator tool was developed under the SGTG project and designed to automate the trip generator estimation process. The purpose of this tool is to enable users to quickly and simply estimate site trip generation for smart growth developments. The tool estimates inbound and outbound vehicle trip generation for typical weekdays when schools are in session, and for AM and PM street peak hours (peak hour between 7-9 a.m. and 4-6 p.m., respectively). The tool helps the user identify and document the analysis site and then qualify the site as being eligible as a smart growth site and appropriate for this method of trip generation estimation. The tool requests several site and vicinity characteristics to determine eligibility. In addition, limited quantitative site data are required for the trip generation computation.

The tool provides the user a simple one page report covering site information, eligibility criteria, input data, and vehicle trip generation estimates.