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941.8 Traffic Impact Study Requirements

Additional Information

● Tips for Reviewing Traffic Impact Studies

The policy of the Missouri Highways and Transportation Commission and MoDOT is to discourage the proliferation of access points and conflict points within the state highway system. For larger developments where the access point requested will meet the guidelines for spacing, a traffic study will be required. To ensure operations on our roadway are not negatively impacted by the additional access, the developer will provide the required roadway improvements.

The amount of traffic generated by a proposed development seeking new or modified access to the MoDOT system is the basis for determining the contents of a traffic impact study. The specific content of a traffic impact study will vary depending on the site and prevailing conditions. At a minimum, contents of a traffic impact study are to include the following major sections, taken from the current Institute of Transportation Engineers (ITE) publication entitled Transportation and Land Development:

1. A description of existing conditions.
2. Estimated trip volume generated by the development and design hour volume for effected driveway(s). These volumes will be based on a method determined acceptable by the district. When ITE trip generation numbers are not appropriate, traffic counts at existing similar locations or other recognized methods can be required.
3. Trip distribution and traffic assignment.
4. Existing versus projected volumes.
5. Capacity analysis for adjacent roadway facilities and for any proposed or existing driveways.
6. Traffic crash analysis for adjacent roadway facilities.
7. Proposed traffic improvements and driveway/access points.
8. Main findings of the study.
9. Summary of findings and recommendations.

For small developments generating fewer than 100 vehicles during the peak hour, or roughly 1000 additional vehicles per day, a traffic impact study is normally not required. However, a review of access location and design is necessary and is to include an analysis of existing conditions, evaluation of sight distance, access design, queuing and site circulation. For any development with access that would qualify as a Hazard Elimination Program location, a traffic study is required to ensure safety is improved along the roadway in conjunction with any access improvements.

For developments generating between 100-500 peak trips, a traffic impact study is required and is to include an analysis of existing conditions at nearby driveways and intersections, crash experience near the site, trip generation, and an evaluation of the number, location and spacing of access points as a minimum.

Developments generating between 500-1,000 peak trips are expected to impact greater distances from the site. In addition to the required information for smaller scale development discussed above, the traffic impact studies for these developments are to consider the future of the roadway, background traffic growth and an analysis of future conditions of nearby intersections or interchanges.

Large-scale development will generally impact the roadway system over a more regional area. A comprehensive analysis is warranted for large developments producing over 1,000 peak trips. Additional information such as mitigation identification and evaluation, gap analysis for unsignalized intersections, analysis of the effect on signal progression and proposed signal locations are included.

For all developments a sight distance evaluation should be completed and meet the criteria described in EPG 941.7 Sight Distance for Entrances. Requests for public street entrances shall meet or exceed both Intersection Sight Distance (see EPG 941.7.1.1 and EPG 233.2.1) and Stopping Sight Distance (see EPG 941.7.1.2). The use of any other sight distance criteria will require authorization from the Central Office Highway Safety and Traffic Division.

At the pre-application meeting, the study limits of the traffic impact study are to include any adjacent intersections that could be impacted. Other specific parameters are set for the traffic impact study, including cycle lengths and operating speeds. A freeway analysis is included with any developments impacting existing or proposed interchanges. Consideration is given to what modeling software can be used for the analysis. The selected modeling software is to be capable of analyzing the systems effects of all impacted intersections and interchanges within the chosen study limits.

A summary of findings and recommendations is part of any traffic study. Developers are responsible for mitigating any unacceptable impacts to the roadway system by the construction of any needed roadway improvements, as indicated by the approved traffic impact study.

Traffic studies may be required by local government organizations. In such situations, MoDOT is to coordinate with the local government and the applicant to ensure one study can meet the needs of all entities.