The following recommendations for traffic count collection, project trip development, and traffic forecasting methodologies have been developed to maintain consistency across different transportation impact studies and reflect current best practices.
Traffic Counts
Data for existing traffic conditions should be collected for the project using these guidelines:
- Peak period turning movement counts at all study intersections, roadway segments (if required), and/or driveways, including bicycle and pedestrian counts at intersections with high non-automotive use. For intersections with a high percentage of heavy vehicles, turning movement counts should separately count heavy vehicles.
- Average Daily Traffic (ADT) for all roadways within the study area (if required) and vehicle classification counts in areas with a high percentage of heavy vehicle use.
- Traffic counts should not be used if they are more than one year old without prior approval from the Traffic Division.
- Traffic data should not be collected during weeks that include a holiday or non-school session periods unless approved by the Traffic Division.
- Traffic data should not be collected between Thanksgiving and the first week of the new year without prior Traffic Division approval.
- Traffic counts should be conducted on Tuesdays, Wednesdays, or Thursdays.
Unless directed otherwise by the Traffic Division, counts should be collected during the following time frames:
- Morning (7:00 a.m. to 9:00 a.m.)
- Afternoon/evening (4:00 p.m. to 6:00 p.m.)
- Midday and “School-Release” peak hours – As directed by the Traffic Division
- Other peak hours, off-peak, weekend, or special events may also be required depending on the project location and type of use
Count data should be included in the study appendices.
Trip Generation
The latest edition of the Institute of Transportation Engineers’ (ITE) Trip Generation should be used for trip generation forecasts. Local data (if available) is preferred but should be approved by the County prior to application. Trip generation for high truck-generating uses such as high cube warehouses, logistics space, etc. shall be determined with County input on a case-by-case basis. The proposed trip generation should be listed in the scoping form for review and approval by the Traffic Division.
Trip internalization for mixed-use developments (if applicable) should be calculated using best practices methodologies. At the time of this memorandum, the EPA’s mixed-use trip generation (or MXD) methodology is considered best practice and should be approved by the Traffic Division prior to use in any studies. Trip internalization calculations (including gross trips, net trips after internalization, and MXD input assumptions such as intersection density, TOD assumptions, acres, etc.) should be documented in the TIS.
For projects that anticipate significant truck traffic, all truck trips should be converted into passenger car equivalents (PCE) for the capacity analysis, or the analyst should adjust the heavy vehicle percentage in the capacity assessment appropriately.
Trip Distribution
A graphical representation of the proposed project trip distribution should be based on the relative location of population, commercial, recreational, and employment centers; existing peak hour link and turning movement volumes; ADT volumes; proximity to regional transportation corridors; knowledge of local and regional traffic circulation; and/or a select zone assignment from SBTAM. A preliminary trip distribution pattern should be submitted in the scoping form for review and approval by the Traffic Division.
The trip distribution may be further refined, after consultation with the Traffic Division, based on the following factors:
- Type of proposed development
- Location and intensity of development
- Conditions on the roadway network in the vicinity
- Similar land use in the vicinity
- Truck route system
- As directed by the Traffic Division
Trip Forecasts
For Cumulative Conditions, SBTAM should be used to develop future traffic volume forecasts. For all projects, the applicant should include cumulative development projects in the study. A list of proposed and approved developments can be obtained from the County’s Land Use Services Planning Division. For proposed projects in close proximity to incorporated cities, those cities should also be contacted to obtain a list of proposed and approved development projects in those jurisdictions. At a minimum, future projects located within two miles of the study area should be included. Projects outside that radius should be included if the traffic generated by that project can reasonably be expected to impact a study intersection and/or road segment.
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