Policy and Procedure
Marion County Department of Public Works
Transportation Impact Analysis (TIA) Requirements
A Transportation Impact Analysis (TIA) evaluates the adequacy of the existing transportation system to serve a proposed development, and the expected effects of the proposed development on the transportation system. The TIA should provide adequate information for County staff to evaluate the development proposal and, when appropriate, recommend conditions of approval.
Throughout the Transportation Impact Analysis process (and beginning as early as possible), cooperation between County staff, the applicant, and the applicant's traffic engineer is encouraged to provide the best possible conditions for the traveling public and potential users of the proposed development, and to reduce TIA report revisions and review time. If County staff can be of assistance in any way during this process, or if any questions arise about this process, please do not hesitate to consult us for clarification or assistance.
Marion County staff may, at its discretion, and depending on the specific situation, require additional study components in a TIA or waive requirements deemed inappropriate. Marion County staff may waive a TIA that would otherwise be required if the developer agrees to certain conditions of development.
Marion County assumes no liability for any costs or time delays (either direct or consequential) associated with Traffic Impact Analysis preparation and review. Marion County Public Works reserves the right to charge an hourly fee to cover staff time for excessive or repeated reviews necessitated by TIA inaccuracies or deficiencies.
When Will A Transportation Impact Analysis Be Required?
A Transportation Impact Analysis shall be required for:
A) Any proposed development that can be reasonably expected to generate more than 600 vehicle trip ends during a single day and/or more than 100 vehicle trip ends during a single hour.
B) Any proposed zone change that, in typical build-out scenarios, can be reasonably expected to generate more than 300 vehicle trip ends more than the previous zoning during a single day
C) Any development within the Urban Growth Boundary of a city if the development would meet that city's criteria for requiring a Transportation Impact Analysis.
A Transportation Impact Analysis may be required for:
A) Any proposed development that can be reasonably expected to generate more than 200 vehicle trip ends during a single day or more than 40 vehicle trip ends during a single hour.
B) Any case in which, based on the engineering judgment of the Public Works Director, the proposed development or land use action would significantly affect the adjacent transportation system. Examples of such cases include, but are not limited to ,: non-single family development in single-family residential areas, proposals adding traffic to or creating known or anticipated safety or neighborhood traffic concerns, or proposals that would generate a high percentage of truck traffic (more than 5% of site traffic).
Calculation Of Trip Generation And Distribution
Trip generation data provided in the most recent edition of the ITE publication Trip Generation should be used unless more appropriate data is available. Average trip generation formulas (where applicable) or rates are normally used; however, more conservative calculations may be required by staff in some cases. Directional trip distribution assumptions should be based on historical data, existing and future travel characteristics, and capacity constraints. County staff may require data collection at similar facilities if County staff determines that insufficient trip generation data is currently available. To reduce revisions and review time, approval of the trip generation and distribution assumptions (including any applicable pass-by, internal, or diverted linked trip percentages) and methodology should be obtained from the Public Works Department before using these assumptions in the Transportation Impact Analysis .
Determination Of The Area For Which Analysis Is Required
The Transportation Impact Analysis shall address at least the following areas:
A) All proposed site access points.
B) Any road segment or intersection where the proposed development can be expected to generate more than 360 additional vehicle trips during a single day or more than 60 additional vehicle trips during a single hour (these typical volumes may need to be adjusted for unusual situations, such as heavy truck traffic, safety issues, or capacity limitations). If a two-way-stop controlled intersection currently functions acceptably and the proposed development would be expected to generate a total of less than 60 additional vehicle trips per day on the minor leg(s) of the intersection, it need not be included in the study area as a result of this requirement. County staff may, at their discretion, choose to waive study of certain intersections if they deem such study to be unnecessary.
C) Any road segment or intersection where the additional traffic volume created by the proposed development is greater than 10 percent of the current traffic volume (for road segments) or the current entering volume (for intersections). Public Works staff may, at their discretion, choose to waive study of certain intersections in some cases.
D) For developments expected to generate more than 30 truck trips per day, the TIA study area shall include the route(s) that these trucks would take from the site to and from the arterial system.
E) Any other intersections adjacent to the subject property.
F) For developments expected to generate a significant percentage of truck traffic (more than 5 percent of site traffic), consult Public Works staff to determine the study area.
G) Any other intersections identified by Public Works staff as having capacity, safety, neighborhood, and/or geometric concerns. Consultation in advance with Public Works staff to determine the extent of the study area is strongly encouraged.
Horizon Year
The horizon year of a Transportation Impact Analysis is the most distant future year that shall be considered in the Transportation Impact Analysis. The horizon year will be a specified number of years after the development opens, and this number will vary depending on the size of the development, any land-use plan changes necessary to allow it, its uses, and the anticipated time until full buildout. The following table shows the TIA horizon year (expressed in years after the development is planned to open) for developments expected to generate less than 5% truck traffic:
Development Type / Trip Generation Per Day Horizon Year
Any Zone Change 20 years
Other Development, Less Than 1,000 0 years
Other Development, 1,000 to 1,999 5 years
Other Development, 2,000 to 4,999 10 years
Other Development, 5,000 or more 20 years
For developments expected to generate more than 5% truck traffic, consult County staff for the TIA horizon year. County staff may, at their discretion, reduce the horizon year in cases where less future study is necessary.
Transportation Impact Analysis (TIA) Report Requirements
The preparer is encouraged to coordinate preparation with County staff and staff from other jurisdictions, as appropriate to ensure that all necessary components are included in the TIA and to reduce TIA revision and review time.
In order to be reviewed, the Transportation Impact Analysis (TIA) report shall include at least the following minimum components (incomplete reports will be returned to the applicant's representative for completion):
1) The TIA report shall be signed and stamped by a Professional Civil or Traffic Engineer registered in the state of Oregon.
2) An executive summary, discussing the development, the major findings of the analysis, and the mitigation measures proposed.
3) A vicinity map showing the location of the proposed project in relation to the transportation system of the area.
4) A complete description of the proposed development, including a site plan, with the best available information as to the nature and size of each proposed use, and the proposed location and traffic control of all proposed access points (including the distance from all proposed access points to adjacent accesses and/or streets).
5) A brief description of the current (and proposed, if applicable) land uses adjacent to the site, including the location, size, zoning, current use, and future use of any land parcels that are not part of the subject application, but may use the subject parcel for all or part of their access. If there is potential for development of these parcels, include the best available information as to the potential future use of each parcel.
6) A description of the TIA study area, including roadway names, locations and functional classifications, intersection lane configuration and traffic control (including signal timing), existing Right-of-Way, transit routes and stops (if any), pedestrian and bicycle facilities, and planned transportation system improvements.
7) Existing traffic volumes (measured during design conditions and/or the peak season within the previous 12 months, unless County staff deems newer counts necessary due to recent development or seasonal variations). Consult County staff to determine what type of count data (turning movement, ADT, or classification) is necessary.
8) Accident data within the study area for the most recent available three year period (accident data can be obtained from the Oregon Department of Transportation).
9) Existing performance of the transportation system, including Levels of Service (LOS) and Volume/Capacity ratios (V/C) for all intersections and road segments as appropriate within the study area.
10) Complete trip generation figures for all aspects of the proposed development, including number of trips by vehicle type and size, and time-of-day and entering/exiting percentages. These figures shall include trip generation figures for any other proposed developments on the subject property, and/or any proposed developments that would share access with the subject property. For developments expected to generate a significant amount of truck traffic (more than 30 trucks per day), include separate figures for trucks. Document the sources of this trip generation data. If the source is other than ITE's Trip Generation, the preparer must obtain approval of the use of such data from County staff before using it in the TIA.
11) Trip generation figures for any pending and approved developments that would affect the study area. County staff will facilitate procurement of applicable data in these cases.
12) Identification of the critical analysis period(s) and justification of this identification.
13) Trip distribution for the proposed development. For developments expected to generate more than 30 truck trips per day, include separate trip distribution figures for trucks.
14) Forecast traffic volumes without the development, in the year that the proposed development is planned to open, and in the horizon year (consult County staff for information to determine these future traffic volumes). If phased development is proposed, include projections for the year that each phase of the development is planned to be complete.
15) Forecast performance (including LOS and V/C) of the transportation system without the development in the year that each phase is planned to be complete and in the horizon year.
16) Forecast traffic volumes, including the proposed development traffic, in the year that each phase of the development is planned to open, and in the horizon year.
17) Forecast performance (including LOS and V/C) of the transportation system, with the proposed development, in the years that each phase of the proposed development is planned to open, and in the horizon year. Include analysis of signal warrants, signal progression, queue lengths, and other traffic flow characteristics as appropriate. For developments expected to generate a significant percentage of truck traffic, demonstrate how the analysis adequately accounts for the presence of these trucks in the traffic flow.
18) Safety analysis of the site accesses, including sight distance and operational characteristics.
19) Analysis of right and left turn lane warrants, queue lengths, acceleration lanes, throat lengths, channelization, and other characteristics of the site accesses as appropriate.
20) Comparison of the location and spacing of the proposed accesses with Marion County standards, the standards of the appropriate city for developments within Urban Growth Boundaries, and/or Oregon Department of Transportation standards for developments near state highways.
21) Analysis of the parking needs of the proposed development, the adequacy of the proposed facilities to meet those needs as appropriate, and the conformance of the proposed parking facilities to applicable standards.
22) Evaluation as appropriate of the turning and traveling characteristics of the vehicles that will be using the proposed development and the adequacy of the geometrics of the existing and proposed roadway (public and/or private) configurations to accommodate these characteristics.
23) Analysis as necessary of the adequacy of the internal vehicle and pedestrian circulation systems to serve the proposed development and how the design of the development addresses the Transportation Planning Rule requirements regarding pedestrian-, bicycle-, and transit-friendly developments.
24) Analysis as appropriate of any potential adverse or controversial effects of the proposed development on the transportation system or quality of life in the area. Examples of possible effects include, but are not limited to, infiltration of non-residential traffic into residential neighborhoods, traffic noise, creation of potential for traffic violations, conflicting turning movements with other driveways, etc.
25) Analysis as appropriate of the effect of the proposed development on pedestrian and bicycle transportation in the area, and any new pedestrian or bicycle transportation needs arising from the development.
26) Listing of all intersections and locations that are projected to not meet Marion County (or other jurisdiction, as appropriate) intersection performance standards in the TIA study area during the required analysis period (see methodologies for Marion County intersection performance standards).
27) Description and analysis of mitigation measures necessary to bring these intersections and locations into compliance with the applicable standards. Include analysis showing that these measures will bring these locations into compliance and include signal, turn lane, or other warrant analyses as appropriate. The TIA shall also specify the timing and phasing of any new traffic signals and the length of any new turn lanes. Any mitigation measures recommended in the TIA shall be physically and economically feasible, and this feasibility may need to be demonstrated in questionable cases.
28) Copies of raw traffic count data used in the analysis (this may be presented in an appendix).
29) Calculation sheets and/or computer software output for all LOS and V/C calculations in the analysis. For signalized intersections, this must include the signal timing used in the analysis (this may be presented in an appendix).
30) Warrant worksheets for signals, turn lanes, signal phasing, all-way-stops, and other proposed measures as appropriate (this information may be presented in an appendix).
Additional Study Requirements
The basic TIA report requirements are listed in the previous section. Additional information and analysis will be necessary to properly analyze many development scenarios, and the Transportation Impact Analysis shall include a complete analysis of the existing conditions and the proposed development. The applicant and/or the traffic engineer can and should submit any additional information that may be helpful to County staff in understanding the proposed development and/or the traffic that it would generate.
County staff may require additional study beyond the scope of the original TIA, especially in cases where additional transportation system concerns arise either as part of the traffic analysis process, as part of the approval process, or from the general public. County staff may also, at their discretion, choose to waive certain report requirements where they deem such analysis to be unnecessary. Please do not hesitate to contact County staff if there is any question as to whether or not certain analysis information should be included in the TIA.
Methodologies and Analysis Parameters
A) All signalized and all-way-stop controlled intersections shall operate at a Level Of Service D or better (all individual movements shall operate at LOS E or better) with a Volume/Capacity ratio of 0.85 or less. Other unsignalized intersections (including unsignalized private accesses) shall operate at Level Of Service E or better, although LOS F may be allowed if the movement has a relatively low volume (as determined by County staff) and there is no indication that a safety problem will be created. Intersections within the Urban Growth Boundary of a city shall also meet the intersection performance standards of that city. Intersections near state highways shall also meet the standards of the Oregon Department of Transportation.
B) Acceptable analysis methods include the most recent Highway Capacity Manual, PASSERII, HRR211, TRANSYT-7F, SIGCAP, and UNSIG10 for most cases. For high percentages of truck traffic, unusual types of intersections, or other cases which do not specifically fit the circumstances for which the above analysis tools are intended, or if the engineer believes that another analysis method more accurately models the situation, consult County staff for determination of the appropriate analysis procedure. Analysis performed using methods not accepted by County staff will be returned to the applicant's representative for revision and correction.
C) Signal timing used in capacity or progression analysis shall use the same cycle length as is currently in use at the intersection, unless specifically noted otherwise, and shall not exceed 136 seconds. Signal timing shall provide adequate available green time (according to Marion County standards) for pedestrian crossing in all directions, and shall provide a minimum of 15 seconds of available green time for protected left turn phases, and a minimum of 10 seconds of available green time for protected/permissive left turn phases. Current yellow and all-red time shall not be decreased.
D) Saturation flow rates greater than 1800 passenger cars per hour per lane shall not be used unless specifically measured at that location.
E) Peak Hour Factors greater than 0.85 shall not be used unless justified by specific counts at that location.
F) Arrival Type 3 (random arrivals) shall be used in signalized intersection analysis unless specific measurements at that intersection indicate otherwise.
G) Signal Progression shall be analyzed in all cases where either a new signal or a change in signal timing is proposed on a roadway with more than two traffic signals (including the new signal, if appropriate) in the space of one mile. A minimum greenband width equal to 40 percent of the cycle length shall be maintained on all arterials, at a progression speed within five miles per hour of the posted speed limit.
H) Any proposed signal timing shall provide adequate green time for pedestrians to cross all legs in all directions, at a speed of 4 feet per second, plus a six-second cushion.
I) All calculations and analysis results should be reasonable, understandable, consistent, and fully explained. Calculations, graphs, tables, data, and/or analysis results that are contrary to good common sense will not be accepted, and may lead to the TIA being returned to the applicant's representative for correction.
J) The conclusions presented in the TIA shall be consistent with and supported by the data, calculations, and analysis in the report. Inconsistent and/or unsupported conclusions will not be accepted, and may lead to the TIA being returned to the applicant's representative for correction.
K) Provide two copies of the Transportation Impact Analysis report for County Staff to review. If any portion of the study area falls within another jurisdiction (such as a state highway or a city), consult that jurisdiction to determine the number of additional copies that they will need for their review.
L) The attached checklist will be used by County staff to determine if a TIA contains sufficient information to be reviewed. Incomplete and/or unacceptable TIAs will be returned to the applicant's representative for completion and/or correction. Acceptance for review does not certify adequacy and is in no way an approval. Additional information may be required after acceptance of the TIA for review.
M) Cooperation between the applicant, the applicant's traffic engineer, and County staff is strongly encouraged throughout the TIA process. The applicant or applicant's traffic engineer should not hesitate to contact County staff if any uncertainties should arise.
See TIA Checklist
