The Anatomy of a Traffic Impact Study

Before a new development can be built, the local government will often require a traffic impact study (TIS) to be performed. But what exactly goes into a TIS?

The purpose of a TIS – also called a traffic impact analysis (TIA) – is to assess the expected traffic impacts to the roadway network surrounding a proposed development. It also recommends improvements that may be necessary to mitigate those impacts. The following are typical steps we follow when performing a TIS.

  1. Site visit – A site visit is normally conducted at the beginning of the project. We inventory existing conditions and note any conditions that could adversely affect traffic within the study area.
  2. Traffic data and existing conditions analysis – We typically collect peak hour turning movement counts, which give the total number of vehicles that travel through an intersection during a given observation period. The peak periods of traffic flow are determined from these counts.  Other traffic data, like approach counts, speed data, and vehicle classification data, could be necessary depending on the development type and location. We use this data to help us understand the existing traffic patterns and any capacity issues. 
  3. Trip generation & assignment – The next step is to determine how much traffic will be generated by the new development. We primarily use the Institute of Transportation Engineers (ITE) Trip Generation manual, which provides data on over 170 different land uses, to estimate the number of trips that the proposed development will generate. The additional trips are divided into “new trips” or “pass-by trips.” New trips are those where the development is the destination, such as an office or school. Pass-by trips are identified as stopping at a development, like a gas station or fast-food restaurant, on the way to another destination. We then assign the trips to the roadway network based on the existing traffic patterns or population distribution in the surrounding area. 
  4. Level of service comparison – Once the generated traffic has been added, we can compare the operational performance of the study area after development to the existing condition, which is called the “level of service” (LOS). An intersection’s LOS is expressed as a letter grade from A to F. In most communities a LOS from A to C is considered desirable, D is deemed acceptable during peak hours, and E & F are unacceptable. In communities with heavy congestion, the acceptable LOS standards may be lower.
  5. Additional analysis – In some cases, further research is required. For example, if an intersection has an unacceptable LOS, it may be appropriate to evaluate the need for a traffic signal or an additional turn lane. While many TIS evaluations focus solely on vehicular traffic, it is best to consider all transportation users (e.g. pedestrians, bicyclists and transit riders) to ensure that all travel modes are appropriately accommodated.
  6. Recommendations – Once all analysis has been completed, we determine any improvement recommendations necessary to mitigate the transportation impact of the development and summarize everything in a report, which is usually reviewed and approved by the department of transportation (DOT) or the local governing body.

If all goes well, we can predict and mitigate traffic issues before they occur, allowing people to arrive at their destinations safely and on time.