Roadway Standards Section 295 - Transportation Impact Study (TIS) Requirements

295.1 General

The objective of a transportation impact study (TIS) is to assess the impacts of a proposed project or land use action on the transportation system and identify mitigation for any capacity or safety deficiencies.  These requirements are intended to provide standards for generation of a TIS for land development applications that are consistent with land use regulations and guidelines for traffic analysis that is prepared for County capital projects.   

295.2 Requirement for a Traffic Impact Study

1. A TIS shall be required based upon an assessment of Engineering regarding the anticipated relative impact of a proposed development on the existing or planned transportation system. 
2. A TIS to address traffic capacity is not required where the proposed development will generate less than twenty vehicles trips in any peak hour unless to address specific safety issues identified by the County.  The need for a TIS is at the discretion of the Road Official.

295.3 Traffic Study Scope and Coordination

1. Engineering and the applicant should coordinate to develop a written TIS scope that will guide the work of the TIS and define the study requirements based upon the anticipated influence area of the proposed development.
2. The influence area of a proposed development establishes the requirements of analysis for the TIS and is defined on a case by case basis, but is typically based upon the trip generation of the proposed development in relation to the proximity of congested roadways and intersections or the proposed development's potential impact on safety issues.

295.4 Traffic Engineering Expertise

All traffic impact studies shall be conducted under the direction of and stamped by an Engineer with expertise in traffic engineering. 

295.5 Coordination with Other Agencies

As applicable, the applicant is expected to coordinate with ODOT and affected local jurisdictions in the scoping, development, and review of a TIS regarding intersections or roadways that are not under the County's jurisdiction.  In some cases, the County has adopted adjacent agency standards as part of the ZDO.

295.6 Zone Changes and Comprehensive Plan Amendments

Zone changes and Comprehensive Plan amendments require analysis compliant with OAR 660-012-0060, the Transportation Planning Rule.  

295.7 Clackamas Regional Center (CRC) Area Analysis Period

1. The ZDO and Comprehensive Plan Chapter 10 require special analysis periods within the Clackamas Regional Center Area. 
2. Within the CRC area, a weekday PM peak two hour analysis is required.  The first hour of analysis shall be based upon the peak hour of the subject intersections.  If the mobility standard is met for the first hour, no further analysis is required.  If the mobility standard is not met in the first hour, a second hour of analysis is required.  The second hour of analysis shall be based upon the "shoulder" ½ hours before and after the peak hour, which may require additional traffic counts.
3. Within the CRC area, a weekday midday hour analysis is required.

295.8 Analysis Methodology

1. All analyses shall be conducted in compliance with the most recent versions of the following:
2. Highway Capacity Manual (HCM) with the exception of signalized intersections, which shall be based upon the HCM 2000
3. ITE Trip Generation Manual
4. ITE Trip Generation Handbook
5. MUTCD
6. AASHTO - A Policy on the Geometric Design of Highways and Streets
7. AASHTO - Guidelines for Geometric Design of Very Low-Volume Local Roads
8. AASHTO - Highway Safety Manual
9. It is recognized that in many instances, the HCM's methodology is limited especially in highly congested conditions.  In instances where the HCM is incapable of providing accurate analysis, the County and applicant shall coordinate on an appropriate analysis method.
10. The following establish the County's LOS and v/c determination method and other analysis parameters required to evaluate the requirements of the Comprehensive Plan and ZDO. 

295.8.1 Two-Way Stop Controlled (TWSC) Intersections

At two-way stop controlled intersections, including driveways, the weighted average methodology of the critical approach of the HCM shall determine the LOS and v/c of the intersection. 

295.8.2 All-Way Stop Controlled (AWSC) Intersections

At all-way stop controlled intersections, the intersection average methodology of the HCM shall determine the LOS and v/c of the intersection. 

295.8.3 Signalized Intersections

At signalized intersections, the intersection average methodology of the HCM shall determine the LOS and v/c of the intersection.

295.8.4 Roundabout Intersections

At roundabouts, the critical approach shall determine the LOS and v/c of the intersection.

295.9 Signalized Intersection Analysis Parameters

1. The TIS shall analyze traffic signals in the following manner with regard to traffic signal timing:
2. The existing, background and total traffic analysis shall be consistent with the existing signal timing. 
3. Analysis of isolated intersections shall optimize the existing splits within the maximum cycle length.
4. Signals that operate in a coordinated timing plan shall be analyzed without adjustments.
5. If modifications to the signal timing are proposed or shown in the analysis, the total traffic analysis shall contain two scenarios:  total traffic with existing signal timing and total traffic with proposed signal timing.  However, signal timing modifications are typically not acceptable as mitigation to a project's impacts.
6. Additional analysis requirements may apply in the case of coordinated signal systems as changes at one intersection may affect other intersections not included in the study area of the project.
7. The Engineer shall request County, ODOT and local jurisdiction signal timing for use in their analysis.  The Engineer is responsible for ensuring that the appropriate signal timing is used in the analysis.
8. If signal timing changes are recommended by the applicant and approved by Engineering as acceptable mitigation to a project's impacts, funds will be required to design and implement new signal timing plans suggested as mitigation by the applicant.
9. Where adaptive signal timing exists, the applicant and County will work together to determine the appropriate signal timing parameters.

295.10 Peak Hour Factor ("PHF")

1. The peak 15 minutes of the peak hour shall be the basis for determining intersection operations except as noted in the ZDO and Comprehensive Plan, which allows a one hour peak period in some situations.  In those instances, the peak hour factor shall be set to 1.0 and the entire peak hour traffic volume shall be evaluated. 
2. The PHF shall be derived from the existing raw manual turning movement counts and be applied uniformly over each scenario except as provided below in 3).
3. In the case of zone change or comprehensive plan amendment analysis, the peak hour factors presented in ODOT's Development Review Guidelines or existing PHF shall be used in analysis.  

295.11 Microsimulation Models

1. Congested conditions will require the use of microsimulation models.
2. The use of microsimulation shall require compliance with the ODOT Analysis Procedures Manual (APM)

295.12 Growth Rates and In Process Traffic

1. For short term analysis of five years or less, linear growth rates shall not be less than 2% per year unless verifiable evidence is submitted or known which indicates that the local growth rate is less than 2% per year.
2. For long term analysis of six years or more, linear growth rates should not be used if regional travel demand volumes are available.  In that case, the analysis should rely upon regional travel demand volumes and post process those volumes per the APM.  If no regional travel demand volumes are available, the historical trends methodology of the APM or other approved methodology may be acceptable.
3. In process traffic, or developments that have been approved and have current land use approval yet are not occupied or fully built-out, shall be included in addition to growth projections.  That information may be omitted for zone changes and comprehensive plan amendments.

295.13 Turning Movement Counts

Turning movement counts shall be conducted as follows:

295.13.1 Count Hours

The count hours stated in Table 2-19 shall be collected in analysis unless the TIS scope specifies otherwise.

295.13.2 Day of Week

Turning movement counts shall be conducted on Tuesdays, Wednesdays and Thursdays unless otherwise directed.

295.13.3 Holidays

Turning movement counts shall not be conducted within one week of a federal holiday.

295.13.4 Current Counts

Traffic counts should be based upon counts collected within 24 months of the completed land use application.  Counts older than 24 months may not be accepted or may require adjustment to current traffic conditions with Engineering approval.

295.13.5 Vehicle Classification, Bicycles and Pedestrian Data

Turning movement counts shall separately include vehicular (truck vs. non-truck), bicycle and pedestrian traffic volumes.

295.14 Trip Generation

1. Trip generation shall be based upon the latest edition of ITE's Trip Generation Manual and Trip Generation Handbook. 
2. The traffic impact study shall include an estimate of site generated trips, pass-by trips, diverted linked trips, and internal capture trips during each study period.
3. If a trip generation rate similar to the proposed use is not available within Trip Generation Manual, then the procedures of the Trip Generation Handbook regarding obtaining local trip rates should be used unless otherwise approved by Engineering.  Engineering may require evidence that the use's trip generation is similar to the use described in the Trip Generation Manual.
4. Trip generation shall be based upon an average weekday unless otherwise specified by Engineering.

295.15 Trip Distribution

Trip distribution shall be approved by Engineering.

295.16 Queuing Analysis

1. All development may be required to provide a queuing analysis that evaluates the impact of queues onto public roadways.  Developments that are anticipated to back traffic onto public roadways are considered to be a safety issue. 
2. Development that proposes a drive-thru service shall provide a queuing analysis that evaluates the impact of drive-thru queues onto public roadways.
3. As required by Engineering, the TIS shall provide 95th percentile queue estimates for each movement at each study intersection.  The TIS shall identify the existing available queue storage, anticipated 95th percentile queue and indicate if that storage is or will be exceeded.
4. In cases where the anticipated 95th percentile queue exceeds the available storage and the queuing is considered to be a safety issue, the development will be required to mitigate a queue back to the background traffic condition.
5. Microsimulation utilized to substantiate queuing shall comply with Section 295.11.

295.17 Traffic Safety

If required, the TIS shall analyze the safety of the transportation network by addressing the following.  If any of these conditions cannot be met and mitigation does not sufficiently address the deficiencies, Engineering may recommend that a proposed development does not meet safety standards:

1. Sight distance is adequate subject to the standards of Section 240.
2. Crash history is adequate per Section 295.17.1.
3. Queuing is adequate per Section 295.16.
4. Access requirements are adequate per Section 220.
5. A turn lane, if warranted and recommended by Engineering per Section 295.18.1, is provided. 
6. Truck circulation is adequate per Section 295.17.2.
7. Off-site access is adequate per Sections 225.4 and 225.5.

295.17.1 Crash History

1. Crash history shall be analyzed for all study intersections and sections of roadway to which access is proposed.  Crash rates, frequency and severity shall be evaluated.
2. The applicant shall evaluate the existing crash history to determine crash patterns, severity and frequency and make recommendations for safety improvements. 
3. Intersection crash rates in excess of typical crash rates require further safety analysis based upon the Highway Safety Manual the APM to determine the development's impact on safety and may require proportional mitigation. 
4. As required, segment crash rates in excess of typical crash rates require further safety analysis based upon the Highway Safety Manual and the APM to determine the development's impact on safety and may require proportional mitigation. 

295.17.2 Truck Circulation

1. For developments that will generate greater than 50 daily vehicles of a size greater than or equal to WB-50, an analysis of truck turning movements at the intersection of the following will be required:
   1. The nearest collector or arterial roadway (whichever is closer) if within the UGB.
   2. With the nearest arterial roadway if outside the UGB.
2. Mitigation will be required if a safety issue would result.

295.18 Mitigation

1. The traffic study shall include suggested mitigation to comply with ZDO and Comprehensive Plan mobility requirements, to provide a minimum level of safety to support the proposed development and to address other requirements of these Standards as required. 
2. Proposed mitigation may require a safety analysis based upon the Highway Safety Manual and APM.
3. Mitigation that shall be considered in the analysis is described below, as applicable:

295.18.1 Turn Lane Warrants

1. The applicant shall analyze the need for right and left turn lane warrants as required by Engineering. 
2. The County utilizes the current ODOT left turn and right turn siting criteria of the Highway Design Manual when establishing the possible need for left and right turn lanes. 
3. The affirmation of a warrant being met for a turn lane does not dictate its installation.  County staff will make a determination regarding the need for turn lanes.
4. Signalized intersections shall be accompanied by channelized left turn lanes on the major street and minor street and designed per Section 250.8.8 except as approved per Section 170.
5. Signalized intersections may require channelized right turn lanes as warranted above and designed per Section 250.8.8.

295.18.2 Traffic Signal Approval

A traffic signal proposed to address safety or capacity issues shall meet the requirements of Section 260 including evidence to indicate that other alternatives have been considered. 

295.18.3 Analysis of Impacts on Local, Residential Streets

1. Some developments may have a detrimental effect upon existing residential uses.  As applicable, the TIS shall evaluate impacts such as traffic volume increases, potential speed increases, safety impacts and other livability issues on local, residential streets. 
2. Based upon the relative impact of the development upon local, residential streets,  improvements to mitigate a development's impact may be required. 
3. Elements to be considered as potential mitigation include the traffic calming measures of Section 265.

295.18.4 Other Mitigation

Other mitigation, such as the installation of stop signs, warning signs, signal timing modifications, additional lanes, roundabouts, traffic circles, pedestrian and bicycle improvements and other potential improvements, will be evaluated on a case by case basis.  Suggested mitigation shall be accompanied by appropriate engineering analysis to allow for the review of such proposals including a review of the mobility and safety impacts of the proposed mitigation. 

295.19 Traffic Study Components

The following elements typically make up a TIS.

295.19.1 Executive Summary

An executive summary that discusses the proposal and the results of the study and any necessary traffic related mitigation to meet the requirements of the land use application.

295.19.2 Project and Study Area Description

A description of the existing and proposed land uses, site characteristics, surrounding land uses, roadway system characteristics, transit service, pedestrian and bicycle facilities, and any pending transportation system improvements as identified in the Clackamas County Capital Improvement Program, Comprehensive Plan, and ODOT or local jurisdiction plans.

295.19.3 Analysis Periods and Scope

1. Analysis of intersection capacity and/or roadway segment capacity, as required by the Scope, meeting the requirements of these Standards, ZDO and Comprehensive Plan. 
2. The analysis shall include the following study scenarios at a minimum or as directed by staff:
3. Existing Traffic (Analysis based upon "current" traffic counts)
4. Background Traffic at a reasonable full build-out year (Existing Traffic + Growth + In Process Traffic)
5. Total Traffic at a reasonable full build-out year (Background Traffic + Site Generated Traffic)

295.20 Submittal Requirements

1. Completed traffic studies and revisions shall be submitted as a pdf document.
2. All electronic files used in support of a traffic analysis shall be submitted upon request.