Traffic Impact Analysis Benefits Everyone (but keep it reasonable)

A traffic impact analysis goes by many names — some of which I feel uncomfortable repeating here. In general however, its purpose is to simply answer the question, “How much traffic will a proposed development generate and what can be done to mitigate unacceptable impacts?”

Because developments range in magnitude from single family subdivisions to large multi-use retail developments, the scope of any traffic analysis should likewise be proportional to the expected impact.  All too often traffic engineers simply apply textbook guidelines to every development without regard for what is reasonable or appropriate.

That being said, typical content for a meaningful, logical, and mutually beneficial traffic study includes the following elements:

Proposed Development — size and type of land use(s) including any future development phases.

Proposed Access — location map showing proposed access relative to existing access driveways and the surrounding roadway network.

Existing Traffic — existing traffic in vicinity of proposed development including key intersections and adjacent roadways.

Expected Traffic — this is based on historical data for the proposed land use and generally includes AM and PM peak hour traffic volumes as well as typical weekday.

Distribution of Traffic — area map depicting the percentage distribution of traffic (ingress and egress) overlaid on adjacent roadway network to include proposed driveways, adjacent roadways and nearby intersections of concern.

Traffic Analysis — this is where the rubber meets the road — Level of Service (LOS) analysis is performed for all proposed driveways and intersections of concern. The analysis includes three basic scenarios: 1) existing traffic, 2) existing traffic plus background growth expected during project build-out, 3) existing traffic plus background growth expected during project build-out plus new traffic expected from the proposed development (LOS analysis is a procedure for calculating congestion, delay, and vehicle queuing and used for comparing access scenarios and evaluating mitigation measures).

Summary of Impacts — based on LOS analysis it is determined where traffic concerns exist and if they are a result of the development (in whole or in part) and how nearby intersections or site access can be modified to benefit ingress, egress, as well as through traffic in the vicinity of the development.

Recommendations – appropriate recommendations are made to mitigate areas of concern and often include new (or longer) turn lanes, traffic signals (or improved phasing or timing), or revised driveway configurations in order to optimize site access and minimize impact to through traffic.

Note: Some jurisdictions have adopted formal guidelines for the preparation of traffic studies. In these cases, a scoping meeting is recommended prior to analysis to agree on the level of analysis appropriate for the proposed project.

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