Best Practices for Traffic Impact Studies

Abstract

For many years there have been concerns that some traffic engineers may approach traffic impact studies with an eye toward assisting developers expedite their development approval rather than delivering an unbiased evaluation of the impact of the development on the surrounding traffic system. Without unbiased studies, agencies are not able to make wise decisions to preserve the capacity of the infrastructure. Alternately, some agencies may also misinterpret the data from traffic impact analyses, resulting in overcharging of developers by imposing requirements for transportation improvements and conditions that are simply not warranted. The goals of this research project were to examine decisions being made from traffic impact studies and to develop a set of best practices to supplement existing guidelines for developing and reviewing traffic impact studies. The research project selected and analyzed 12 case studies to compare post development traffic conditions to the traffic impact study forecasts of post implementation traffic conditions. Results of these comparisons were mixed. Best practices were identified for the following areas: land use code selection and application; pass-by trip reduction assumptions; seasonal variations; evaluation of other modes; analysis software; regional demand model versus growth rates; future year analysis; and safety.

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