7.0 Analysis Process
This section outlines the traffic impact analysis process.
7.1 Scoping
Consultation with City staff is essential when preparing a TIA. The consultant must submit a scoping letter to the City detailing the project and including the following information:
- Project Description
- Existing and proposed land uses
- Project Trip Generation
- Study Intersections and Roadway Segments (if required)
- Project Trip Distribution
- Project Trip Assignment
- Analysis Scenarios and Methodologies
The City will review the provided information and discuss the analysis requirements with the consultant. The study area or other parameters may be adjusted by the City at this stage. A copy of the approved scoping agreement should be included in the study appendices. See Appendix A for the Scoping Agreement for Traffic Impact Study.
7.2 Trip Generation
The project's trip generation must be based on the latest edition of the ITE Trip Generation Manual. Approval from the City is required before using other data sources. Pass-by and diverted link trip calculations can be conducted based on the Trip Generation Manual and should be compared to traffic counts on adjacent roadways to verify if existing traffic can support the pass-by reductions. For mixed-use projects, internal trip capture can be based on either the Trip Generation Manual or the traffic model.
For industrial uses, ITE trip generation rates must be converted to PCE trips based on vehicle splits from the Truck Trip Generation Study prepared by the City of Fontana. Passenger car and truck trips must be identified separately, as well as the total PCE trips.
Unknown Trip Generation Rates: For unique trip generators, a trip generation survey might be required. Some unique development types or uses may not have rates/formulas published by ITE. In such cases, a trip generation study may be conducted at a similar existing facility to determine acceptable trip generation rates for the study. The type and location of the similar existing facility and the study methodology must be pre-approved by the City Engineer.
7.3 Trip Distribution
Project trip distribution should be discussed with City staff. For projects generating more than 250 peak hour trips, the trip distribution must be based on a traffic model. Truck traffic distribution should not be based on the traffic model since other factors, such as truck routes, play an important role in truck routing.
7.4 Background Volume Development
7.4.1 Existing Traffic: Existing traffic counts should be conducted on a Tuesday, Wednesday, or Thursday on non-holiday weeks. Counts near schools should be taken when the school is in session. For the analysis, traffic counts should be converted to Passenger Car Equivalents (PCEs) based on vehicle classification counts conducted at least one intersection at each CMP facility. The following PCE conversion factors should be used:
- 2-Axle Trucks: 2.0 PCE
- 3-Axle Trucks: 2.5 PCE
- 4- and more Axle Trucks: 3.0 PCE
Intersections without classification counts should use a factor of 2.5 PCE for all trucks. Traffic volumes at adjacent intersections with inconsistent traffic counts (more than 3% variance in approaches and departures) should be balanced with the higher traffic volume approach to account for inconsistencies in traffic counts. For freeway mainline volume development, traffic counts from the most recent Caltrans Counts database should be used. Truck traffic should be converted to PCEs based on a factor of 1.5 for all trucks.
7.4.2 Forecast Traffic: All traffic forecasts should be based on PCEs. If a traffic model is used, forecast link volumes should be identified in PCEs. Standard model post-processing techniques consistent with SANBAG methodologies should be used, and then converted to turn volumes based on NCHRP-255 methodologies.
7.5 Level of Service Analysis
Level of Service (LOS) analysis should be based on HCM methodologies, with input parameters consistent with the Highway Capacity Manual. Key parameters include:
| Input Parameter | Value |
|---|---|
| Base Saturation Flow Rate | 1900 pc/hr/ln. |
| Heavy Vehicle Factor | Based on traffic volumes if PCE conversion not conducted. If PCE conversion conducted, then 0%. |
| Cycle Length | 60-130 seconds |
| Minimum Green Time | 10 seconds (for through movements only). In high pedestrian areas, minimum green times should be based on CAMUTCD walk time calculations. |
| Lost Time | 2 seconds per phase |
| Peak Hour Factor | Based on counts for existing and near-term (less than 5 years) conditions. 0.95 for later scenarios (unless existing PHFs are higher). |
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