The purpose of this project is to update the 2009 Traffic Impact Study (TIS) and to expand the TIS to include all developments.  The TIS guidelines will help the member agencies evaluate the potential impact of proposed and potential future development have on the street network and the operations of controlled intersections.  The guidelines will assist in Identifying in advance any potential adverse impacts on the existing transportation network and ensure adequate mitigation is provided for by proposed development.  Other purposes include:

• Assist public and private sector entities in the early identification of issues related to traffic operations, including but not limited to driveway/access locations, traffic signals, and other elements of transportation facilities; and
• Support long term planning solutions that foster responsible growth of transportation infrastructure consistent with the local government’s Comprehensive Plan and vision for the community.

Problem Statement

The geography in the Portneuf Valley limits the number and width of the collectors and arterial network.  The 2009 plan addressed the medium-sized commercial developments on collectors and arterials. The regions need to determine the impact of smaller developments and the cumulative impact local streets can have on the collector and arterial network.   Small developments on local and collector streets combined can have a significant impact on the operation and design requirements of the network.   Capacity or operational issues can be caused by not accounting for the potential impact development will have on the network.  A lack of understanding of the impact of development on the network has created unidentified infrastructure needs that are difficult to address after the development has occurred.

Project objectives

The Traffic Impact Study guidelines have ten objectives; they are:

1. Update the 2009 guidelines to current standards and references;
2. Identify the Long-Range Transportation Plan, Comprehensive plans, and Idaho Code, which supports Traffic Impact Studies.
3. Identify the procedures for estimating total traffic, which consists of existing traffic, growth in existing traffic, and site-specific traffic. Total traffic analysis will consider the following: site access driveway(s), level of service and other measures of effectiveness including traffic safety, bicycle/pedestrian considerations, traffic control needs, and roadway Improvements/traffic management;
4. Develop a multi-tiered TIS threshold for both commercial and residential developments;
5. Develop the analysis zone for each level of development. The analysis zone is the area and development to include in the analysis;
6. Identify site-specific development impacts to will include in the study;
7. Identify cumulative impacts of planned future developments on the study location;
8. Establish service volumes and Level of Service Criteria for each class of roadway;
9. Evaluate the currently accepted techniques to evaluate the impact of development on the street network. At a minimum evaluate the ability, data requirements, and ease of use of the Highway Capacity Manual, Synchro, TRAFFIX and TransModeler
10. Develop content for the TIS and a fillable format report template for each of the recommend development levels.

Project Coordination

A project development team will assist staff and consultant in developing the TIS guidelines. The team should include members from:

• City of Pocatello (engineering, streets, and planning)
• City of Chubbuck (engineering and planning)
• Bannock County (planning)
• Idaho Transportation Department
• Developers
• Consultants
• Land use commission members

Tasks

The tasks are proposed to meet the project objectivities.

Task 1. State of the practice review

A comprehensive review of the current traffic impact requirements from regional and national communities of population size as the BTPO planning area. A review of the recommended practices from major research sources such as Transportation Research Board, Institute of Transportation Engineers, AASHTO, Others.

The review should include the following elements:

• What types of developments require a TIS?
• What tools are used to evaluate traffic impacts?
• What traffic elements are analyzed
• When is development required to conduct a TIS and what types of TIS are required? (size thresholds)
• Referenced national guidelines or manuals
• Identified standards including capacity, service level, turn lane, and auxiliary lanes requirements

Task 2.  Identify streets and traffic control features to evaluate

Identify the features to be evaluated and evaluation criteria.  Features include intersections control, operation, and analysis.

Task 3. Identify the Level of service (LOS) standards, service volumes, and requirements for traffic control needs and other measures of effectiveness.

The task includes both street segment and intersection standards. For the street, identify the service volumes and LOS standard for each class.  Service volumes are the level where the specific roadway goes over the LOS criteria

Task 4.  Evaluate and analyze identified analysis tools

Identify the tools used in analyzing impacts and evaluate them on the following criteria

• Requirements
• Ability to evaluate the selected design and service standards
• Ease of use and ability for developers to use the tool.

Conduct a case study on several different intersections and road class types.  The case study will collect actual data from problem and non-problem intersections.  Compare the criteria results from each of the models and methods identified.  At a minimum evaluate Highway Capacity Manual Software, Synchro, TRAFFIX, and TransModeler.

The data should allow the evaluation of items identified in tasks 2 and 3.

Task 5. Identify Levels of Traffic Impact Study

Determine the requirements and levels ranging from minimal impact to anticipated high impacts.  For each TIS level, the outline of the TIS along with the anticipated findings of each level should identify. 

Task 6. Identify procedures to determine the cumulative impact of developments

Many developments alone do not impact the arterial network alone, but developments that use the same access street may cumulatively impact the arterial and on the adjacent intersections to the planned development.  The task will identify a process to determine the cumulative impact of all developments included in the Traffic Analysis Zone of the development.  

Task 7. Evaluate the potential of a region-wide TIS for all collectors and arterials

Using the travel demand model and community viz software, evaluate key intersections with the criteria identified to evaluate developments.

Task 8. Identify other potential issues, including traffic safety, bicycle/pedestrian considerations, traffic control needs, and roadway Improvements/traffic management.

Task 9. Develop an outline of TIS Document

The task will develop outlines for each required TIS type.  

Task 10. Traffic Impact Study Guidelines Report

The task will develop a traffic impact study guideline.

Task 11. Coordination with ITD TIS Guidelines

The state highway is governed by a different standard than the current 2009 guidelines.  Historically, if the project is on the state highway system, ITD guidelines applied.  This method worked when the number of trips trigger was the same.   A process for addressing state highways needs to be developed.

A project development team will assist BTPO staff in developing the TIS guidelines. The team should include members from:

• City of Pocatello (engineering, streets, and planning)
• City of Chubbuck (engineering and planning)
• Bannock County (planning)
• Idaho Transportation Department
• Land use commission members

 State of the practice review

A comprehensive review of the current traffic impact requirements from regional and national communities of population size as the BTPO planning area. A review of the recommended practices from major research sources such as Transportation Research Board, Institute of Transportation Engineers, AASHTO, Others.

The review should include the following elements:

• What types of developments require a TIS?
• What tools are used to evaluate traffic impacts?
• What traffic elements are analyzed
• When is development required to conduct a TIS, and what types of TIS are required? (size thresholds)
• Referenced national guidelines or manuals
• Identified standards including capacity, service level, turn lane, and auxiliary lanes requirement

State of the Practice Analysis

Guideline review

Traffic Analysis Tools Recommendation

Identify the tools used in analyzing impacts and evaluate them on the following criteria.

• Requirements
• Ability to evaluate the selected design and service standards
• Ease of use and ability for developers to use the tool.

Conduct a case study on several different intersections and road class types.  The case study will collect actual data from problem and non-problem intersections.  Compare the criteria results from each of the models and methods identified.  At a minimum, evaluate Highway Capacity Manual Software, Synchro, TRAFFIX, and TransModeler.

Capacity and Level of Service Standards

Level of Service Analysis

Intersections

A level of Services (LOS) Analysis should be conducted on all study intersection and commercial driveway using the Highway Capacity Manual Software (Synchro or maybe another yet to be determined)

Signalized Intersection – The acceptable LOS for signalized intersections is LOS D for collectors with no approach lower than LOS E.  For arterials, the standard is LOS E for the intersection with no approach blow LOS LOS E for Arterials.

Signalized Intersections (Table 7.2 Transportation Impact Analysis

Source: HCM Version 6.0 Exhibit 19.8

Un-signalized Intersection  – The acceptable LOS for an un-signalized intersection is LOS D for any leg of the intersection.  A signal warrant analysis should be conducted on any intersection of collectors and arterial where the LOS of any leg is below LOS E.

Unsignalized Intersections (Table 7.3) Transportation Impact Analysis

Source: HCM Version 6.0 Exhibit 20.2 (Two-Way Stop) and Exhibit 21.8 (All-Way Stop)

Street Segments

Service standards are divided into non-arterial and arterial streets.  The services standards are approximate  LOS equivalent for different traffic volumes levels and are used to determine the recommended street sizing and classification.

• The capacity numbers are for planning use only and should not be used to make design decisions.
• The Level of Service is not usually calculated for local streets. The services volumes are derived from many sources, including Highway Capacity manual Exhibit 16-16 LOS C form two-lane signalized roads. With a K-Factor of 0.09.
• Simplified Highway Capacity Calculation Method for the Highway Performance Monitoring System Table 16.

Intersection Design Elements

Access Control

Access control for commercial driveways is covered in the existing Access Guidelines.

Intersection Design (curb radius and sight distance

The local jurisdiction determines the design of intersections.

Road Placement Standards

The design of intersections is determined by the local jurisdiction or the master street plan.

Traffic Impact Analysis Guidelines The draft document revises the process to conduct a traffic impact analysis and includes three levels of analysis.