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Analysis of Impacts – City of Walnut Creek Transportation Impact Analysis

8.0 ANALYSIS OF IMPACTS

8.1 Proposed Project Trips

Trip Generation

The estimate of trip generation should follow industry-standard practices, including but not limited to:

  • Institute of Transportation Engineers (ITE) Trip Generation Manual (latest edition):

    • Peak hour trips should be based on adjacent street traffic peaks.

    • Utilize weighted average rates or regression equations per ITE guidelines.

    • Special considerations for antiquated data, small sample sizes, or high standard deviations (>110%) require additional data collection.

  • Alternative Data Sources: If ITE data is unavailable or inadequate, sources such as San Diego Association of Governments (SANDAG) or locally collected trip generation surveys may be used with appropriate documentation.

  • Downtown Pedestrian Retail District: Commercial land uses (except office spaces) should apply ITE Trip Generation rates for Shopping Center (Land Use Code 820) at 1.5 million square feet:

    • Daily: 26.32 trips per 1,000 sq. ft.

    • AM Peak Hour: 0.54 trips per 1,000 sq. ft.

    • PM Peak Hour: 2.45 trips per 1,000 sq. ft.

Trip Reductions

Subject to City staff review, reductions in project trip generation may include:

  1. Existing Trips: Trips from existing land uses that will be replaced should be deducted, based on actual observations rather than ITE estimates.

  2. Internally Captured Trips: Applies to mixed-use developments with complementary land uses that allow walking or biking trips.

  3. External Trips: Includes pass-by trips (limited to specific land uses such as retail, restaurants, gas stations) and diverted linked trips (should not be deducted).

  4. Multi-Modal Trips: Reductions may apply for projects within one-half mile of transit, subject to documented justification:

  • Up to 10% reduction for sites within one-quarter mile of BART.

  • Up to 5% reduction for sites between one-quarter and one-half mile from BART.

  • Further reductions for Transportation Demand Management (TDM) strategies must be justified.

Trip Distribution

A figure should illustrate the percentage of project-generated traffic assigned to the transportation network. Trip distribution should consider:

  • Proposed land use and existing travel patterns.

  • Access to major corridors.

  • Contra Costa Transportation Authority (CCTA) travel demand model for general trip distribution (manual assignment should be used for local intersections and roadways).

  • Accounting for turning restrictions, congestion levels, and relevant roadway characteristics.

8.2 Traffic Analysis Scenarios

LOS and other analyses should be conducted for the following scenarios:

  • Existing Conditions – Current traffic volumes and facilities.

  • Existing + Proposed Project Conditions – Project trip generation added to the existing scenario (included in Appendix only).

  • Near-Term (Without Project) Conditions – Approved and pending project traffic added to existing volumes.

  • Near-Term + Proposed Project Conditions – Proposed project trips added to Near-Term Conditions.

  • Cumulative Year 2035 (Without Project) Conditions – Based on CCTA projections adjusted to reflect 2035 growth.

  • Cumulative Year 2035 + Proposed Project Conditions – Project trips added to cumulative projections.

Phased developments should include analysis for each development phase’s full occupancy year. Transportation facility improvements may only be assumed if programmed in the City’s Capital Improvement Program (CIP).

8.3 Performance Measures

All intersections and roadways must meet Level of Service (LOS) standards in the City of Walnut Creek General Plan. If below standard, they are considered unacceptable.

LOS Analysis Methodologies

  • Signalized Intersections: CCTA LOS method using Synchro or City-approved software.

  • Unsignalized Intersections: HCM 2010 methodology, using HCS, TRAFFIX, Synchro, or City-approved software.

  • Roundabouts: Analyzed using SIDRA, with additional simulation required for interactions between closely spaced roundabouts.

  • Freeways: HCM 2010 operational analysis per Caltrans TIS Guidelines.

  • Routes of Regional Significance: Analyzed using Delay Index and Average Speed methods per CCTA Technical Procedures.

Signal Warrants

  • California MUTCD Peak Hour Traffic Volume Warrant (latest edition) shall be applied.

Presentation of Results

Findings should be presented in tables summarizing:

  • Volume-to-Capacity (V/C) ratio

  • Delay (in seconds per vehicle)

  • LOS for each analysis scenario

  • Delay Index and Average Speed for regional roadways

All analysis assumptions must be cited and justified for reproducibility by City staff or peer consultants.

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