Mixed-Use Development (MXD) Trip Generation
Current methods of traffic impact analysis, which rely on rates and adjustments from the Institute of Transportation Engineers (ITE), are believed to understate the traffic benefits of mixed-use developments (MXDs) and potentially overstate potential roadway impacts because they don’t fully account for the interactions between land uses or the surrounding built environment. A national study for the US EPA, performed by a team composed of Fehr & Peers and academic researchers developed a new methodology to more accurately predict the traffic impacts of MXDs. The study evaluated household travel surveys from 239 mixed-use developments in Seattle, Portland, Sacramento, Houston, Atlanta and Boston and found statistical relationships between site characteristics and the amount of vehicle travel generated in and out of the site. MXDs were found to reduce traffic impacts relative to single-use suburban development, due to the following key factors.
- Diverse on-site activities that capture a large share of trips internally.
- Placement within walkable areas with good transit access that generate high shares of walk and transit trips.
- Central locations that reduce trip lengths.
The potential vehicle trip reductions from MXDs were significant enough to demonstrate that conventional trip generation methods could exaggerate roadway impacts and the need for mitigation including higher impact fees, exactions, and negotiated payments than should be the case. These factors could also contribute to potentially discouraging development of otherwise desirable projects.
Demonstrated Validity of MXD Method
The MXD method was developed based on data from 239 nationwide mixed-use developments, and has been validated based on traffic counts at 28 other MXDs in various parts of the country. Six of those 28 sites are in South Florida and are presented in the ITE Trip Generation Handbook. Other sites are located in California, Utah, Texas, Georgia and North Carolina. The sites represent a wide range of densities, land use mixes, and development scales.
The table below compares the statistical performance measures for three methods:
- Trip generation rates for each individual land use contained in the mixed-use site based on publications such as ITE Trip Generation and SANDAG Traffic Generators.
- Conventional methods of discounting for mixed-use internalization based on ITE Trip Generation Handbook or SANDAG reduction factors.
- Vehicle trips computed with the MXD method.
The statistical validation measures indicate that the MXD model performed better than the other methods and had the smallest level of error.
- The studies described here were commissioned by the US EPA and San Diego Association of Governments (SANDAG).
- The study was led by Reid Ewing at the University of Utah and Jerry Walters at Fehr & Peers. Special thanks to Michael Greenwald, Ming Zhang, Mackenzie Watten, Robert Cervero, Lawrence Frank, and John Thomas for their guidance and contributions to the analysis.
For information, please contact:
Jerry Walters, Principal
Ron Milam, Principal