Traditional Use of Traffic Impact Studies

Development places demands on the transportation system. New development may generate enough traffic to cause congestion, safety concerns, and/or the need for infrastructure improvements—such as new signalization, turn lanes, or crosswalks. Traffic impact studies (TIS) are a common planning tool used by departments of transportation, such as DelDOT, to foresee demands on the transportation network and determine transportation improvements that may be necessary to accommodate new development. Traffic access and impact studies are also intended to maintain a satisfactory level of service and the appropriate access provisions for a proposed development.

Transportation improvements in progress

Credit: James Pernol, DelDOT

When are Transportation Impact Studies (TIS) Required?

DelDOT’s Standards and Regulations for Subdivision Streets and State Highway Access defines a TIS as “a study conducted during the development approval process to determine the impacts that traffic generated by the proposed development will have on the surrounding street network and the improvements needed to the transportation system in order to mitigate those impacts.” In Delaware, a TIS may be required by DelDOT, a local government, or both. Although DelDOT is responsible for most of the state’s transportation network, a local government may require a TIS to determine whether a proposed development is appropriate for a site and context of land use. A TIS may be required as a condition for land use approvals to ensure that area transportation facilities operate adequately to accommodate impacts of new development.

The range of detail and complexity of a TIS may vary—depending on the type, size, and location of the development. While some Delaware jurisdictions require TIS as part of their rezoning and conditional use processes, they are most commonly, and perhaps most effectively, required as part of the subdivision and land development plan processes. Also, DelDOT and Kent and New Castle Counties have warrants for when a TIS is required.

The extent of a TIS is determined in a “scoping meeting” that includes representatives from DelDOT, the developer and, usually, the local jurisdiction. The TIS process generally includes a review of base traffic conditions, an analysis of expected trip generation, an assessment of future traffic volumes, and an analysis of site access locations and nearby intersections. A TIS scope may also consider impacts to queuing at intersections, safety, and impact on other transportation modes (e.g., bicycle, pedestrian, and public transit).

Limitations of a TIS as a Transportation Planning Tool

While a TIS is a valuable transportation planning tool, it is considered a more reactive approach for several reasons. First, a TIS focuses on the need to mitigate transportation impacts of single, new development (e.g., a subdivision) instead of providing a proactive strategy to plan for and fund long-term, area-wide transportation improvements. Second, in many cases, smaller or incremental developments, which cumulatively can have significant impacts, may not trigger the need for required transportation improvements. This may cause an unfair distribution of responsibility where the last developer in pays for all transportation improvements. Third, while a TIS estimates the number of trips generated and projects future traffic volumes, these forecasts are not necessarily incorporated into plans for future land use (e.g., comprehensive plans and master plans). Finally, the TIS approval process can be tedious and time consuming, particularly when multiple developers are involved.

Transportation improvements in progress

Credit: James Pernol, DelDOT

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