What is a traffic impact study?

Local News

Kev/Livingston Parish News

(Livingston Parish News) – As new residential developments continue to spring up in Livingston Parish, questions arise as to how local governments can control the growth.

If at all.

During those conversations, it’s been brought to the public’s attention that the parish has several ordinances on the books to ‘protect the health, safety, and welfare’ of the parish’s citizens.

Those ordinances deal with required building, and sanitation codes, but also include a traffic impact study, as well as a drainage impact study.

As long as a developer or builder does not request a variance on the first piece, and passes both impact studies, there is very little legislative action the parish council has to stop a subdivision from being built.

Chapter 125 of the parish’s code of ordinance deals with subdivisions. Section 20 outlines the ‘traffic impact study’ required for any particular subdivision.

Any development which is considered a ‘subdivision’ must participate in the traffic impact study. However, each development will require an individual study based on a tiered system, which is measured by ‘number of trips during peak hours.’

The only exception to this rule is when a traffic impact study must be submitted to the Department of Transportation and Development for review, usually when a new subdivision is intended on or near a state highway.

Required for all traffic impact studies are the following:

  1. The study must be completed and submitted with the preliminary plat or Stage 3 submittal
  2. All analysis must be performed by a state-registered professional civil engineer with experience in performing similar-type studies.
  3. Prior to beginning a comprehensive traffic impact study (Threshold 3), the engineer will meet with the review engineer and the Planning Director to develop the exact scope of the study and determine the actual area to be studied and methods used.

As mentioned by members of the parish council, rarely – if ever – do they see a traffic impact study with the preliminary plat. As was discovered, publicly, during the Premier Concrete issue, the parish administration “typically” handles issues with individual projects once it has passed the planning commission and the parish council.

Most traffic impact studies come during the aforementioned third phase of correspondence between the developer and the parish.

The council can, however, request that the results of both the traffic impact study as well as the drainage study be brought back before the council before final approval.

There are four thresholds that will affect which level of traffic study will be applied to an individual subdivision. According to the ordinance, the number of trips are determined from data and study of a similar subdivision, or a “trip generation manual” which is created by the Institute of Transportation Engineers and generates comprehensive data relative to land-use codes and their percentage traffic flow during certain times of day.

However, the raw numbers attached to those percentages are not given by the manual. Civil engineers have to draw data from the average number of vehicles per household.

For subdivisions of less than 40 trips per day, the following applies:

  • The proposed trip generation and distribution
  • Source of information (trip generation manual), or a comparison to an equivalent site in the Parish with known trip data
  • Sight distance evaluation.

For subdivisions which would produce 40 to 75 trips a day, the following applies:

  • (Requirements for a subdivision with 40 trips or less) AND
  • Analysis of the access/egress of development in relation to level-of-service of the adjacent roadway system.

For any subdivision that would have between 75 and 400 trips a day, the following applies:

  • (Requirements for a subdivision with 41-75 trips) AND
  • Recommendations for any roadway and/or intersection improvements to maintain or improve the existing level of service
  • Provide vehicle accident data in proximity to site (if available)
  • Analysis of the roadway capacity (existing and/or proposed) on all roadway links abutting the proposed development site and identify necessary roadway and/or intersection improvements to maintain the existing level-of-service
  • An analysis of the nearest major intersection (typically signalized) in each direction from the major site driveway.

For any subdivision with 401 trips or more per day, the following applies:

  • (Requirements for any subdivisions of smaller trip value) AND
  • The new traffic generated for the proposed development would be distributed onto the existing transportation network within an area defined by the engineer review agency. Analysis of each roadway link and intersection link within this area for both existing and proposed conditions
  • Identification of any deficiencies determined by this analysis and resolve such deficiencies.

The Foxglove subdivision, a 300+ home D.R. Horton development off of Duff Road north of Walker, pushed the traffic impact study to require a light be installed at Burgess Avenue and Duff Road between the 67th and 68th home. Parish Councilman Tracy Girlinghouse pushed a cooperative endeavor agreement between the developer, the parish, and the city of Walker stating that no occupancy permits would be issued for the 68th home or further until D.R. Horton constructed the required intersection light and turning lanes.

However, road width is not a common factor in the traffic impact study. Many roads are 18 feet wide, Chad Bacas with Forte & Tablada told the council in November, and that has not affected a traffic impact study yet.

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