Trip generation is usually based on ITE’s latest publication of the

Trip Generation Manual

or other acceptable sources. ITE’s

Trip Generation Manual

includes equations for daily and study peak hour trip rates based on a specified independent variable. In many cases, this can be the square footage of the specific land use, or the number of units contained within a residential development. Example trip generation calculations can be found in

Appendix Q, Section 3 – TIA Example Problems.

Volumes are usually generated with average rates or a fitted curve equation. Consult the ITE Trip Generation Handbook Chapter 4 “Trip Generation Manual Data” to decide whether rates or equations are used.

Vehicle trip reduction may be applicable depending on the project location and type of land uses in a proposed development. If used, it is recommended this reduction follows the methodology found in the ITE

Trip Generation Manual

If reducing vehicle pass-by trips, the percentage reduced typically equates to the total vehicle trips generated minus the internal capture used for each land use. Pass-by percentage has an upper limit percentage of the existing vehicular traffic to be set while scoping with TxDOT.

Project distribution of the proposed trips is assigned using engineering judgment. If only one project driveway is proposed, all trips enter and exit through this driveway. This process is used to create a trip distribution and traffic assignment exhibit.

When analyzing the future roadway network, background growth and future project traffic is added to the existing traffic counts. Background growth data may be available from the city or MPO. Historical AADT information can be averaged for at least the last five years of data, when available, to obtain an average annual growth rate to grow the background volumes. Using this annual growth rate, existing traffic is grown to the future study years specified in the scope.

If there are other approved developments to be included in the study within close vicinity to the proposed project, traffic can be projected for each development through the process of trip generation and added to the roadway network through trip distribution and traffic assignment.

The combination of grown traffic and traffic from other approved developments are typically combined into a background traffic scenario and provided in an exhibit. This is typically completed for each future study year.

New traffic created from the proposed site is added to the background traffic scenarios, resulting in a background plus site traffic scenario for each study year. Peak hour volumes for these scenarios are usually provided in separate exhibits for each scenario.

The future traffic analysis includes all geometric characteristics of the existing analysis with the addition of proposed driveways. Background traffic and background plus site traffic scenarios are typically analyzed.

Future levels of service are obtained for all study intersections, along with any new project driveways used to access the project development. The latest HCM analysis methodology is typically used. For signalized intersections, each approach and the overall intersection are analyzed. For stop-controlled intersections, each stopcontrolled approach is analyzed by the approach. Left-turning movements of non-stop controlled approaches are typically analyzed by movement.

Traffic signal timings are typically adjusted and optimized in future scenarios in order to improve LOS. This can also be part of the proposed improvements. TxDOT area offices can request that analysis be run with existing timings in order to preserve existing signal coordination. If an intersection is operating over capacity, the mitigation requirements in Section 4 can be reviewed and used to help with capacity issues

An

example

Traffic Analysis can be found in

Appendix Q, Section 3 – TIA Example Problems.

TxDOT’s Access Management Manual and RDM

(

Appendix Q, Section 4 – External References ‘Reference 4’

) is used for driveway spacing and turn lane configuration

Chapter 8 “Freeways” of the RMD has information on developments near freeways that can impact a TIA.

If the future scenario analysis shows high or unacceptable delays, appropriate mitigation measures are taken to correct or alleviate the issues on the roadway network. If a signalized intersection is experiencing unacceptable vehicular delays, phasing and timing settings are optimized to mitigate the delay levels. If signal optimization is not enough to alleviate the vehicular delay levels, improvements to intersection geometry or type are considered. If an unsignalized intersection experiences notable delay, redistribution of trips to other routes is considered.

Review the location of access points on site in relation to existing nearby access points and intersections for safety considerations. It is recommended that the site plan be examined to confirm the design of external access points account for bicycle and pedestrian safety. It is recommended that a safety and crash analysis based on the past five calendar years be conducted within the study area. Compare crash rates for the study area with statewide/municipality crash rates. Identify CRF to mitigate key locations. See

Appendix Q, Section 4 – External References (Reference 6, 7)

for additional information regarding safety analysis for Traffic Impact Analyses. Key safety questions to consider while conducting a safety analysis for Traffic Impact Analyses include:

What is the historical crash analysis in the study area?
What are the existing crash types involving fatalities and serious injuries? Where are those crashes? What road user types are represented in those crashes?
Will the project create additional multimodal conflict points?
Will the project result in increased vehicular speeds?
Will the project lead to longer queues that would affect through traffic and cause speed differentials?
Based on crash estimation methods from the HSM methodologies, will the project potentially contribute to more crashes? Review locations that will be modified by the project and evaluate designs.
What countermeasures would improve safety in the study area? Incorporate them in the project to improve safety.
Is there a safety plan in place for the study area? If so, integrate recommendations from the safety plan in the project
Are the crash rates in the study area above the statewide averages for similar roadways? Identify safety-focused mitigation measures.

Consider right-turn deceleration lanes when there are large numbers of right-turning vehicles into the project site. TxDOT’s Access Management Manual outlines thresholds for right-turn lanes at new site driveways. This may involve ROW purchase at corners. See for the threshold values. An

example

of auxiliary lane analysis is shown in

Appendix Q, Section 3 – TIA Example Problems

.

Left-turn deceleration lanes are typically considered for roadways with raised medians. For undivided roadways, reference the TxDOT RDM for left-turn lane volume thresholds and geometry.

Lengthening of storage lanes

Median modifications are considered for both safety improvements and operational improvements. Traffic and safety analysis is performed to determine locations for directional median openings as an alternative to full access openings

As traffic is increased and redirected based on new developments, any existing traffic signals are analyzed for operational deficiencies as well as placement changes that may help with LOS and traffic flow. Preferable traffic signal spacing is one-half of a mile.

If the future demands along the nearby roadway segments are significantly impacted and the roadway is predicted to operate over capacity, widening can be considered to mitigate the extra vehicular traffic volume. ROW limitations and existing geometries are considered when widening is proposed for a project. Improvements, such as a right-turn lane, may necessitate additional ROW.

Consider traffic signal installation at unsignalized intersections with high or unacceptable vehicular delay. In these circumstances, complete a traffic signal warrant analysis following the process outlined in

Chapter 10

of this manual.

Revised striping

Turning lane restrictions

Consider alternative intersections in place of a typical traffic signal intersection. Alternative intersection analysis can be performed following the process outlined in

Chapter 10

of this manual. Roundabout analysis is presented in

Chapter 12

of this manual.