When analyzing future vehicular traffic, it is necessary to account for the background growth that naturally occurs along major roadway segments. The growth rates are based on a combination of the historical AADT and current field count data collected along the segment analyzed. TxDOT’s Traffic Count Database System, linked in

Appendix Q, Section 4 – External References (Reference 1)

, may be used to obtain historical traffic counts. An

example

of calculating background growth rate is provided in

Appendix Q, Section 3 – TIA Example Problems

.

Trips are typically generated for other approved developments near the site. Coordination with TxDOT and governing municipalities takes place to confirm other developments near the site are accounted for in the background vehicular traffic volume growth. Trips for these approved developments are typically generated in the same manner as outlined in the trip generation section below

Any future planned roadway improvements near the project site are considered when developing the future scenario analysis. (Ex. Resurfacing, widening, ITS, traffic signals, etc.) The following resources can be checked for future TxDOT planned projects: TxDOT’s Project Tracker, found in

Appendix H, Section 1 – External References (Reference 6)

TxDOT’s UTP, found in

Appendix Q, Section 4 – External References (Reference 3)

The most common source for trip generation is the Institute of Transportation Engineer’s (ITE’s) latest publication of the Trip Generation Manual. Trips are generated using either the fitted curve equation or average rates provided. It is recommended that the fitted curve equation be used when a fitted curve equation is provided, and the data plot has at least 20 data points or when a fitted curve equation is provided, and the data plot has an R2 value of at least 0.75. Consult the ITE Trip Generation Handbook Chapter 4, “Trip Generation Manual Data,” when deciding between the fitted curve equation or average rates. Daily, AM, and PM peak hour trips are typically included in the analysis. It is recommended that TxDOT be consulted when alternate forms of trip generation are used.

Trip generation for the peak hour of the adjacent street is used for the majority of land uses. For uses with a known non-standard peak, such as a school, trip generation may be done for the peak hour of the generator. Use engineering judgement and consult the local TxDOT District in these cases.

If reductions are to be taken for internal capture, reference the ITE Trip Generation Handbook. This is especially common for mixed-use developments. Internal capture accounts for any trips between land uses within the same development that do not touch the off-site street system.

Example

calculations of trip generation and internal capture can be found in

Appendix Q, Section 3 – TIA Example Problems.

Pass-by trips consist of trips that are already traveling on the adjacent roadway network but enter the proposed development on their way to another destination. This is typically a percentage of the total gross external trips generated. Commercial developments are eligible for pass-by trips. This percentage is based on historical pass-by data collected and is found in the ITE Trip Generation Handbook.

Multimodal reduction is typically used on trip generation in areas where alternate forms of transit are readily available. Use engineering judgement in these situations, and data backing the decision (e.g., ridership numbers) is recommended.

Traffic assignment for future developments are created along the surrounding network. Use engineering judgement and ITE guidance to distribute project vehicular traffic at and around the project site. If using DTA, project traffic assignment will not be needed as it is inherently provided by the model. Distribution results are typically reviewed and approved by TxDOT or the applicable city/county. Historical trends observed along the major roadway segments are considered when distributing the project traffic.

Example

problems involving trip distribution and traffic assignment can be found in

Appendix Q, Section 3 – TIA Example Problems

Roadway improvements are dependent on the project and anticipated trips. If the existing roadway network at and around the project site accommodates all future traffic, roadway improvements may not be necessary. If it is determined that future traffic results in unacceptable operations, propose roadway improvements to mitigate for the deficiencies. Typical off-site improvements are listed under Mitigation Requirements in

Section 16.4