TRB 2016 Blue Ribbon Committee
16th National Transportation Planning Applications Conference

Evaluating the transportation impacts of multifamily housing

Corresponding Author: Amanda Howell, Portland State University

Presented By: Kelly Clifton, Portland State University


Many places in the U.S. are facing significant housing shortages, particularly for low- and middle-income residents, and local and state agencies alike are grappling with how best to respond to this issue. Increasing the stock of multifamily housing is an important policy lever, but the development of such housing is challenged by transportation impact policies and trip generation methodologies that are insensitive to the built environment, socio-economic conditions, and non-automobile modes. Currently, practitioners rely primarily on trip generation rates published by the Institute of Transportation Engineers (ITE), which are based largely on data obtained at suburban locations and recent studies question the accuracy of these data (Clifton et al., 2013; Millard-Ball, 2015). Additionally, some agencies are beginning to rely more heavily on other complex performance measures such as vehicle-miles-traveled (VMT). For example, California is starting to evaluate VMT when analyzing the transportation-related impacts of proposed land use projects as a result of the passage of Senate Bill 743.

An examination of the transportation impacts of residential land uses, including affordable multifamily housing and developments with transportation demand management strategies (TDM) aimed at curbing demand for automobile travel, will lead to a better understanding of such impacts and, hopefully, better estimates. More accurate estimates will help to remove barriers that currently impede the development of multifamily housing, thus assisting jurisdictions in achieving important environmental, social, and economic goals. Drawing upon studies conducted in Oregon and California investigating trip generation, vehicle ownership rates, and VMT, we intend to improve the means of estimating the transportation impacts of new multifamily development and advance multimodal transportation data and methods.

Clifton, K.J., Currans, K. M. and Muhs, C.D. (2013) "Evolving the Institute of Transportation Engineers' Trip Generation Handbook: A Proposal for Collecting Multi-modal, Multi-context, Establishment-level Data," Transportation Research Record: Journal of the Transportation Research Board, Vols. No. 2344 Travel Demand Forecasting, Vol. 2, pp. 14 107-117.

Millard-Ball, A. (2015) "Phantom Trips: Overestimating the traffic impacts of new development," Journal of Transport and Land Use, Vol. 8, No. 1, 1-19.


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