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 from the latest edition of the Transportation Research Board Highway Capacity Manual (HCM) should be used to evaluate the operation of the project driveways and nearby intersections. For individual isolated intersection analysis, the use of software packages such as Synchro, Vistro, or HCS that implement the HCM methodologies is acceptable.
Where oversaturated conditions currently exist, the operational analysis should be conducted using Synchro/SimTraffic or VISSIM simulation models to more accurately reflect the effect of downstream congestion on intersection operations. VISSIM should be used in areas with transit lanes or with high levels of pedestrians conflicting with vehicle turning movements
Los Angeles County Traffic Study Guidelines 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/signalized intersections that are adjacent to the project or that are expected to be integral to the project’s site access and circulation plan,
- Additional intersections may be necessary as determined by Public Works.
For most projects, analyze traffic for both the a.m. and p.m. weekday peak hours as determined by 24-hour traffic counts. For some projects, expanding the analysis to include midday or weekend periods may be appropriate 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 be conducted per the following guidance and by Section 4.1.4.2., unless otherwise directed by Public Works:
- Turning movement data at the study intersections:
- Should be collected in 15-minute intervals,
- Must include vehicle classifications, pedestrian volume counts, and bicycle counts,
- Must include a minimum or 2 hours of traffic counts for each of the peak hours,
- Must be taken on Tuesdays, Wednesdays or Thursdays,
- Must exclude holidays, and the first weekdays before and after the holiday,
- Must be taken on days when local schools or colleges are in session,
- Must be taken on days of good weather, and avoid atypical conditions (e.g., road construction, detours, or major traffic incidents),
- Traffic counts used from other traffic studies in the area may be use if they are reviewed and approved by Public Works.
When simulation analyses are to be conducted, obtain traffic speed and/or travel time data during peak periods to aid in calibration of the simulation model.
Los Angeles County Traffic Study Guidelines Section 4.1.4.4. - Project Trip Distribution
Distribution patterns for project trips should be determined considering a number of factors including, but not limited to, the following:
- Characteristics of the street system serving the project site,
- Level of accessibility of routes to and from the proposed project site,
- Locations of employment and commercial centers,
- Locations of residential areas.
The Transportation Impact Analysis shall include map(s) showing project trip distribution percentages (inbound and outbound) at the study intersections, and project driveway(s). This map shall be pre-approved by Public Works and included in the Transportation Impact Analysis Scoping Document.
Los Angeles County Traffic Study Guidelines Section 4.1.4.5. - Traffic Forecasts
The Transportation Impact Analysis shall estimate traffic conditions for the study horizon year selected during the scoping phase and recorded in the executed Scoping Document. The study shall clearly identify the horizon year and annual ambient growth rate used for the study. For development projects constructed in phases over several years, the Transportation Impact Analysis should analyze intermediary milestones before the buildout and completion of the project. The annual ambient growth rate shall be determined by Public Works staff 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 Transportation Impact Analysis shall consider trip generation for known development projects within one-half mile (2,640 foot) radius of the farthest outlying study intersections. Consultation with the Department of Regional Planning or other planning agencies will be required to compile a related projects list.
The 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 that are expected to be implemented on or before the project buildout year should be identified in the study, in consultation with Public Works. If programmed improvements include a modification to the existing lane configuration at any of the study intersections, then the study should identify these changes and include the revised lane configuration in the LOS calculations for all future scenarios.
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