Section 3: Engineering Analysis

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Engineering studies or analyses can be used to assist in the evaluation of future access connections to the state highway system. In many cases, such as low volume or rural access connections, an engineering study will not be needed. For locations where TxDOT is the permitting authority, the need for an engineering study, and the level of detail, will be determined by TxDOT. In the case of a dispute resolution, the Design Division can request an engineering study and specify the level of study detail.

The purpose of an engineering study is to determine the safety, mobility, and operational impacts that the access connection will have on the highway system. While not applicable to TxDOT, municipalities may require that such studies also determine the compatibility between the proposed land use and the transportation network.

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Early Coordination

As early as possible in the development process, applicants are encouraged to meet with the local TxDOT district staff, and the municipality if applicable, to discuss specific requirements associated with obtaining access to the state highway system. This meeting, in addition to bringing all affected parties together regarding access connection issues, will also help to define the requirements of any needed engineering study.

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Concurrence with Local Guidelines

If the proposed development is within a jurisdictional boundary and the municipality has engineering study or traffic impact analysis guidelines in place, then the applicant is required to adhere to the municipality's guidelines.

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Questions to Consider

When determining the need for and level of detail of an engineering study, the following questions should be considered:

  • Do the proposed driveway(s) meet the minimum spacing requirements per Tables 2-1 and 2-2 (or local requirements, as applicable)?
  • Will the proposed driveway(s) require a deceleration or acceleration lane? If so, refer to the TxDOT Roadway Design Manual for lengths and other design criteria.
  • Are there any sight distance or physical obstructions that will result in a safety problem?
  • Are there any environmental or hydraulic issues associated with the proposed driveway(s)?

The responses to the above list of questions will assist in determining the level of detail required in an engineering study.

If necessary, specifics regarding needed level of study, time of day analysis, phasing of development, and project area can be defined and agreed upon at the initial coordination meeting. Additional information and analysis may be required if the access connection cannot meet the minimum spacing requirements, or there is an operational or safety impact.

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Engineering Study versus Traffic Impact Analysis (TIA)

A Traffic Impact Analysis (TIA), the requirements of which are described below, may be required when a permit for an access connection or the sale of TxDOT controlled access is requested. The following section outlines the purpose and requirements of an engineering study and a TIA.

In nearly all other cases where the access requirements set forth herein are satisfied, a TIA will not be required. Typically, the impacts of an access point along a state facility can be ascertained by means of an engineering study that indicates the forecasted turning movements at the proposed access connections. The forecasted turning movements, used in conjunction with the TxDOT Roadway Design Manual, will determine the need for and the required length of left-turn and/or right-turn deceleration lanes.

Requirements for Engineering Studies and TIAs

The intent of this section of the Access Management Manual is to identify the possible criteria for engineering studies and TIAs. It is by no means meant to minimize the need for the applicant to meet with the local TxDOT district staff to determine the study's requirements. It is the intent of TxDOT to require only those elements of an engineering study or TIA that are necessary to answer the specific questions that arise during the permitting process for specific access points. It is not the intent of TxDOT to require an exhaustive TIA for every application for a driveway permit on a state roadway. The early coordination meeting, as discussed above, will be the mechanism to identify whether or not an engineering study or TIA is necessary and, if so, the level of detail that will be required.

Engineering Study. Should an engineering study be required, it may include the following elements: trip generation, trip distribution, and traffic assignment at the proposed access points. Additionally, the engineering study may require that existing traffic volume data be collected.

The trip generation will be conducted using the latest edition of the Institute of Transportation Engineers Trip Generation manual unless there is acceptable data that supports the use of another trip generation source. Trip distribution will be performed with input from the local TxDOT district staff (and the municipality, if applicable). The traffic assignment will be conducted to determine the forecasted turning movements attributable to the proposed development. The existing traffic counts will be grown using an annual growth rate as agreed to by the local TxDOT district staff (and the municipality, if applicable) to the build-out year of the proposed development. As an example, if the proposed development will take two years to construct and occupy, the existing traffic volumes will be grown by the agreed upon growth factor for two years. The resulting traffic volumes will be used as background traffic volumes, and the assigned forecasted turning movements will be added to the background traffic volumes resulting in the total traffic volumes.

The total traffic volumes will be used to determine the need for left-turn and right-turn lanes. If such lanes are needed, refer to the TxDOT Roadway Design Manual to determine their lengths and other design criteria.

TIA. In the rare instances where a TIA is required by TxDOT, it may include the above mentioned elements as well as the same type of data for intersections adjacent to the proposed site (specific study limits to be defined by TxDOT). Additionally, the TIA may require operational analyses (including LOS and capacity analyses) for the study intersections as determined during the initial meeting between the applicant and the local TxDOT district staff. Furthermore, the applicant's TIA should include recommendations for mitigation measures should the impact of the proposed access point(s) on the state highway system result in unacceptable levels of service.Examples of Levels of Engineering Studies

This section presents examples of scenarios under which an engineering study or TIA would likely be required by TxDOT and the level of detail that would be needed to address the issues associated with the requested access connection. These scenarios are for illustration purposes only and should not be used as thresholds for study level requirements.

The first scenario involves a request that meets the driveway spacing criteria, but is a major development that consists of more than 200,000 square feet of retail development along with associated pad-type developments. Even though the driveway spacing criteria (as defined herein) have been met, it is important for TxDOT to understand the impacts that this large development will have on the adjacent roadway network and the intersections adjacent to the site. The parameters of the engineering study or TIA would be defined by TxDOT based upon the characteristics of the existing traffic, the major intersections relative to the site access, and other operational or safety concerns. Additionally, the engineering study or TIA would likely examine multiple phases of development, assuming that the entire site will not be developed at one time. The phased study or TIA would enable TxDOT to determine the necessary mitigation measures for each phase of development and the specific improvements that should be in place to accommodate the development's traffic. As stated previously, the intent of a TxDOT required engineering study or TIA is not to determine the compatibility of the land use with the surrounding area, but rather to determine the impact of the development and its associated traffic volumes on the state roadway.

The second scenario involves the application for a driveway for a small development such as a single residential unit, single retail unit, or similar land use. The driveway spacing requirements set forth herein are satisfied by the applicant. The existing traffic volumes along the state roadway are relatively low. Neither an engineering study nor TIA would be required in this scenario.

A third scenario would be the application for a driveway for a moderate-sized development that meets the spacing criteria outlined herein, but that raises questions about the proper length of a right-turn deceleration lane as well as the need for a left-turn lane. The local TxDOT district staff may require an engineering study to examine the issues at hand. The applicant would need to provide forecasted turning movement volumes at the subject driveway location as well as background traffic volumes that will also pass through the intersection. These forecasted volumes, along with the state roadway's design speed, can then be used in conjunction with the TxDOT Roadway Design Manual to determine if a right-turn deceleration lane and/or left-turn lane is needed. If it is determined that a left-turn lane is necessary, an operational analysis can be performed by the applicant to determine the appropriate length of the left-turn lane.

The fourth scenario involves an application for a driveway that does not meet the spacing requirements set forth herein. If necessary, TxDOT may request an engineering study or TIA to determine the operational impacts of the proposed driveway on the existing state roadway and adjacent driveways or intersections. The level of detail of this study or TIA will be dependent upon the intensity of the traffic expected to be generated by the planned development. The study may include trip generation, distribution and assignment, but may also include operational analyses at the proposed driveway and the adjacent intersections and driveways. Further analyses may be necessary to determine the operational and safety impacts of the sub-standard spacing on the overall roadway system.

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