Traffic impact analysis & mitigation planning

Traffic impact analysis (TIA) and mitigation planning are specialized engineering studies that forecast the transportation impacts of proposed real estate developments. They support in proposing strategies that can eliminate or reduce the projected impacts.

Key characteristics




WSDOT regions

Other names

  • Traffic impact study

Strategy description

Traffic impacts and their potential need for mitigation are important for any community to consider with new real estate development proposals. Public policy makers, citizens, and developers all have a stake in understanding and responding to additional demands on the transportation system. Traffic impact assessments (TIA) and mitigation measures are the primary way jurisdictions ensure that developments accommodate expected increases in the number of vehicles on nearby roadways without creating undue burden on the transportation system.

TIAs and mitigations are typically mandated by municipalities for developments expected to generate a number of trips above a defined threshold. They provide a structured way for developers to present their developments’ expected transportation impacts to ensure that transportation will be well-served into the future.

TIAs and mitigation planning require planners to forecast the transportation impacts of proposed developments and propose strategies for eliminating or reducing the projected impacts. The assumptions that planners make in a TIA determine the eventual transportation infrastructure at a development. Poor assumptions can lead to future transportation issues that include congestion and safety concerns.

A TIA should answer the following fundamental questions:

  • How will future traffic conditions look with the development compared to without it?
  • Can the existing transportation system accommodate the additional traffic generated by the planned development?
  • What additional transportation needs are required to maintain existing service levels?
  • What roadway improvements are recommended to accommodate the expected development traffic?

Mitigation Measures

Coordination and collaboration is required between the lead agency, other state and local agencies, and the developer preparing the TIA in order to reach consensus on how to address the transportation impacts. National and State environmental laws, known as the National Environmental Policy Act (NEPA) and State Environmental Policy Act (SEPA) processes, specify how potential project impacts are to be disclosed. Eliminating or reducing impacts to a level of insignificance is the standard pursuant NEPA and SEPA. The lead agency is responsible for administering the NEPA and/or SEPA review process.

Typical mitigation measures include:

  • Channelization, such as turn lanes or raised islands
  • Installation of a roundabout or a traffic signal 
  • Donation of right of way
  • Addressing any roadway design or operational deficiencies created by the proposal
  • Possible restrictions of turning movements
  • Sight distance enhancements
  • Payments to a programmed transportation project

When to use this strategy

TIA and resulting mitigation makes sense for any development project that is expected to generate new trips.

Strategy benefits:

  • Provides an unbiased assessment of the expected transportation impacts of developments in the near term and the future
  • Ensures developer accountability for providing transportation mitigations needed to support their developments
  • Brings together various stakeholders—developers, policymakers, transportation officials, and citizens—to reach consensus on what solutions are most appropriate for the region

What you need in order to implement

Policy needs:

  • Agreement on required performance criteria and measures of effectiveness (MOE) to be used to assess traffic impacts. Examples include FHWA’s MOE List
  • Adopted standards for analysis and tools to be used to quantify traffic impacts. Examples include specific versions of the Highway Capacity Manual (HCM), FHWA Traffic Analysis Toolbox, Traffic Analysis Procedures Manual, and WSDOT Vissim Protocol
  • Formal published guidance for the content required in a TIA report. Examples include WSDOT’s Traffic Analysis Guidebook and Development Services Manual

Planning needs:

  • Understanding of the existing transportation system locally and regionally in terms of infrastructure, capacity, and restraints
  • Forecasts of general future transportation demand to predict the transportation impacts of a development
  • Mitigation measures to accommodate traffic from a development to ensure a sufficient level of service
  • Periodic TIA updates, or when traffic does not match predictions, to continually plan for expected future transportation needs

Coordination needs:

  • Coordination with the lead agency, other state and local agencies, and the developer to ensure compliance with local TIA report requirements, including level-of-service and scope of analysis

About key characteristics

Location notes:

TIA and mitigation planning may be applicable for any real estate development project location.

Cost notes:

Costs associated with TIA and mitigation planning for the agency are low as they are typically covered by the developer.

Technology notes:

Estimating future travel demand requires traffic analysis and modeling tools.

Collaboration notes:

Collaboration is required between the developer and any transportation agencies with roads or infrastructure within the impacted area of the development.