Data Collection, Trip Generation, and Forecasting Methodologies
To ensure consistency and accuracy in Transportation Impact Analyses (TIAs) prepared for the City of Calimesa, the following methodologies for traffic data collection, trip generation, and traffic forecasting must be followed.
Traffic Data Collection
Traffic counts form the foundation of a robust TIA. For existing traffic conditions, turning movement counts must be collected at all study intersections, applicable roadway segments, and driveways. Where there is a high volume of non-auto use, pedestrian and bicycle counts should also be included. For areas with a high percentage of heavy vehicles, those vehicles should be counted separately from general passenger cars.
Average Daily Traffic (ADT) counts and vehicle classification data should be gathered for relevant roadways within the study area, particularly where heavy vehicle use is anticipated. Traffic counts should generally be no older than one year. Use of older counts must receive prior approval from the City Traffic Engineer.
Traffic data should not be collected during holidays or non-school periods unless specifically approved by the local agency. Additionally, data collection should avoid the time period between Thanksgiving and the first week of the new year due to atypical travel patterns. Standard data collection should occur on Tuesdays, Wednesdays, or Thursdays to reflect typical weekday traffic conditions. In congested conditions, queue lengths should be documented by approach and movement in 15-minute intervals. Counts should never be taken during active construction near or within the study location.
Unless instructed otherwise, traffic counts should be collected during the following periods to ensure coverage of peak demand:
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Morning peak: 7:00 a.m. to 9:00 a.m.
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Evening peak: 4:00 p.m. to 6:00 p.m.
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Midday and school-release peak: if directed by the City Traffic Engineer
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Additional periods such as weekends, off-peak hours, or special events may be required depending on the development type or site context.
Collected traffic data must be documented and included in the TIA appendix.
Trip Generation
Project trip generation estimates should be based on local survey data from at least three comparable sites, following the procedures in the Institute of Transportation Engineers (ITE) Trip Generation Handbook. If local data collection is not feasible, standard ITE trip rates may be used; however, analysts must clearly disclose the limitations of the ITE data—particularly regarding land use context and geographic variability.
High truck-generating uses, such as logistics and warehouse facilities, must coordinate with the City Traffic Engineer to determine appropriate trip generation methodologies. These should be identified early through the Project Scoping Form.
For mixed-use projects, trip internalization must be calculated using approved state-of-practice tools such as the EPA’s MXD (Mixed-Use Trip Generation) methodology or ITE’s mixed-use methodology. Analysts must document all assumptions, including gross and net trip estimates and MXD inputs (e.g., intersection density, TOD characteristics, acreage).
If significant truck traffic is anticipated, truck trips should either be converted to Passenger Car Equivalents (PCEs) for analysis or incorporated using an adjusted heavy vehicle factor in the capacity analysis. Coordination with the City Traffic Engineer is encouraged to ensure the use of appropriate PCE values.
Trip Distribution
Trip distribution must reflect expected travel patterns based on the project’s location and land use. Preferred methods include mobile device data from at least three similar sites and the use of select zone assignments from regional models such as RIVTAM or RIVCOM. Additional factors such as proximity to employment and commercial centers, existing traffic volumes, and regional circulation patterns should help inform distribution assumptions.
A preliminary trip distribution map must be included in the Project Scoping Form for agency review and approval. The final distribution may be refined based on:
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Project type and intensity
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Roadway network conditions
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Local land uses
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Proximity to designated truck routes
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Agency direction
Traffic Forecasting
Cumulative traffic forecasts must be developed using the adopted Riverside County travel demand model (RIVTAM/RIVCOM) and reflect the cumulative horizon year. Before running the model, the analyst should confirm that the assumed land use allocations reflect approved projects, the City’s General Plan, and local zoning
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