Los Angeles County Traffic Study Guidelines Section 4.1.4. - Methodology
Los Angeles County Traffic Study Guidelines Section 4.1.4.1. - Level of Service and Queueing Methodology
Intersection level of service (LOS) and queueing methodologies should adhere to the latest edition of the Transportation Research Board Highway Capacity Manual (HCM) to assess the operation of project driveways and nearby intersections. Acceptable software packages for analyzing individual isolated intersections include Synchro, Vistro, or HCS, which implement HCM methodologies.
For areas with existing oversaturated conditions, use Synchro/SimTraffic or VISSIM simulation models to more accurately reflect the impact of downstream congestion on intersection operations. VISSIM should be specifically used in areas with transit lanes or high levels of pedestrian activity conflicting with vehicle turning movements.
Section 4.1.4.2. - Study Area
Study locations should be determined in consultation with Public Works and should include:
- All primary project driveway(s).
- Unsignalized and/or signalized intersections adjacent to the project or integral to the project’s site access and circulation plan.
- Additional intersections as determined necessary by Public Works.
Typically, analyze traffic for both a.m. and p.m. weekday peak hours, based on 24-hour traffic counts. For some projects, it may be appropriate to expand the analysis to include midday or weekend periods if these are expected to be the prime periods of trip generation for the project.
Los Angeles County Traffic Study Guidelines Section 4.1.4.3. - Traffic Counts
Traffic counts should generally follow these guidelines unless otherwise directed by Public Works:
- Turning movement data at the study intersections should:
- Be collected in 15-minute intervals.
- Include vehicle classifications, pedestrian volume counts, and bicycle counts.
- Include a minimum of 2 hours of traffic counts for each peak hour.
- Be taken on Tuesdays, Wednesdays, or Thursdays.
- Exclude holidays, and the weekdays immediately before and after holidays.
- Be taken on days when local schools or colleges are in session.
- Be taken on days of good weather, avoiding atypical conditions (e.g., road construction, detours, or major traffic incidents).
- Traffic counts from other traffic studies in the area may be used if reviewed and approved by Public Works.
- When conducting simulation analyses, obtain traffic speed and/or travel time data during peak periods to aid in model calibration.
Los Angeles County Traffic Study Guidelines Section 4.1.4.4. - Project Trip Distribution
Project trip distribution patterns should be determined by considering factors such as:
- Characteristics of the street system serving the project site.
- Level of accessibility of routes to and from the project site.
- Locations of employment and commercial centers.
- Locations of residential areas.
The Transportation Impact Analysis should include map(s) showing project trip distribution percentages (inbound and outbound) at the study intersections and project driveway(s). These maps should be pre-approved by Public Works and included in the Transportation Impact Analysis Scoping Document.
Los Angeles County Traffic Study GuidelinesSection 4.1.4.5. - Traffic Forecasts
The Transportation Impact Analysis should estimate traffic conditions for the study horizon year selected during the scoping phase, as recorded in the executed Scoping Document. The study should clearly identify the horizon year and the annual ambient growth rate used. For development projects constructed in phases, the analysis should include intermediary milestones before project buildout. The annual ambient growth rate will be determined by Public Works during the scoping process and can be based on the most recent SCAG Regional Transportation Model or other empirical information approved by Public Works.
The analysis should consider trip generation for known development projects within a one-half mile (2,640 foot) radius of the farthest outlying study intersections, requiring consultation with the Department of Regional Planning or other planning agencies to compile a related projects list.
Traffic forecasts for the project access and circulation constraints are determined by adding project-generated trips to future base traffic volumes, including ambient growth and related projects, and conducting the operational analysis.
Any programmed and funded transportation system improvements expected to be implemented by the project buildout year should be identified in the study, in consultation with Public Works. If these improvements include modifications to existing lane configurations at any study intersections, the study should identify these changes and include the revised lane configuration in LOS calculations for all future scenarios.
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