TO: Board of Supervisors
FROM: Steven Lederer, Director of Public Works
REPORT BY: Syed Ahsan Kazmi, Senior Traffic Engineer, Public Works
SUBJECT: Napa County Left-Turn Lane Warrant/Screening Update
RECOMMENDATION
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Adoption of a Resolution updating the Napa County Left-Turn Lane Warrant/Screening Update (No Fiscal Impact).
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BACKGROUND
Napa County currently utilizes a left-turn lane warrant methodology to evaluate the need for left-turn lane installations at unsignalized intersections on the County roadway system. Left-turn lanes are an important roadway feature that can improve intersection operations, reduce vehicle delay, and enhance safety by separating turning vehicles from through traffic.
As traffic conditions, roadway safety considerations, and transportation engineering practices continue to evolve, the County periodically reviews its evaluation methodologies to ensure they remain technically sound, consistently applied, and aligned with current industry standards. Recent applications of the County’s warrant methodology, together with advancements in national guidance related to left-turn lane operations and safety, provided an opportunity to assess the effectiveness of the existing criteria and determine whether updates were warranted.
To support this effort, Napa County retained Fehr & Peers to review the County’s existing left-turn lane warrant methodology, evaluate current research and agency practices, and identify potential modifications or replacement criteria. The review included a comprehensive evaluation of national research and guidance documents, with particular emphasis on the National Cooperative Highway Research Program (NCHRP) Report 745, Guidance for the Design and Application of Shoulder and Turn Lanes on Two-Lane Rural Highways. NCHRP Report 745 represents the most current nationally recognized guidance for evaluating left-turn lane needs and is referenced by the American Association of State Highway and Transportation Officials (AASHTO) Green Book, the primary national reference for roadway geometric design.
Throughout the study, staff from the Planning, Building and Environmental Services (PBES) and Public Works Department (DPW) participated in the evaluation and review process. Following completion of the draft report, it was circulated to PBES and DPW staff and revised based on internal comments. The draft report was also distributed to 24 external stakeholders, including traffic engineers, planners, and attorneys involved in the review of development projects within Napa County. Feedback received from both internal and external stakeholders was evaluated and incorporated into the final report, as appropriate.
Based on its findings, Fehr & Peers recommends replacing the County’s existing methodology with screening criteria derived from NCHRP Report 745. The recommended approach provides a modernized, data-driven framework for evaluating left-turn lane improvements and offers several key advantages:
• Alignment with Current Industry Practices - Incorporates Highway Safety Manual crash prediction models to quantify the anticipated safety benefits of left-turn lane installations.
• Improved Technical Accuracy - Utilizes peak-hour traffic volumes and turning-movement data rather than relying primarily on average daily traffic volumes, resulting in a more accurate assessment of operational and safety conditions.
• Greater Consistency and Transparency - Applies clearly defined screening thresholds that reduce subjectivity and provide a more predictable evaluation process for County staff, applicants, and decision-makers.
The proposed action updates Napa County’s left-turn lane screening process for unsignalized intersections on County roadways. The revised process uses peak-hour traffic conditions and turning-movement volumes to assess operational and safety needs while continuing to incorporate engineering judgment and site-specific considerations. Project applicants will be responsible for providing the required traffic data and analysis, subject to review and approval by Napa County Public Works.
If approved, the updated screening process will be incorporated into the Napa County Traffic Impact Study Guidelines and used to support consistent, transparent, and technically defensible evaluations of left-turn lane improvements on County roadways. The Left-Turn Screening User Guide, dated June 2, 2026, is included solely as a reference document and is not subject to approval as part of this action.
Preparation of the Napa County Left-Turn Lane Screening Update Report was funded through the Roads Division budget. No additional funding is required for implementation of the updated screening process.
Requested action:
Adoption of a Resolution establishing Napa County Left-Turn Lane Warrant/Screening Update
FISCAL IMPACT
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Is there a Fiscal Impact? |
No |
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Is it currently budgeted? |
No |
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Is it Mandatory or Discretionary? |
Discretionary |
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Discretionary Justification: |
Based on a review of national guidance and sensitivity analyses of local case studies, staff recommends updating the Napa County left-turn lane warrant to adopt the methodology outlined in NCHRP Report 745. This approach reflects the most current nationally recognized guidance, is endorsed by AASHTO, and is supported by Highway Safety Manual-based safety and benefit-cost analyses. |
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Is the general fund affected? |
No |
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Future fiscal impact: |
None |
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Consequences if not approved: |
If the recommended update is not approved, Napa County will continue to use the existing left-turn lane warrant methodology, which may not fully reflect current nationally recognized guidance and best practices for evaluating left-turn lane needs. |
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Strategic Initiative: |
Build healthy, connected communities. |
ENVIRONMENTAL IMPACT
ENVIRONMENTAL DETERMINATION: The proposed action is exempt from the California Environmental Quality Act (“CEQA”) under the General Rule because it can be seen with certainty that there is no possibility the proposed action may have a significant effect on the environment and therefore the CEQA is not applicable. [See Guidelines For the Implementation of the California Environmental Quality Act, 14 CCR 15061(b)(3).] Future discretionary projects will be evaluated based on site specific conditions to determine whether installation of a left turn lane is required and if so, the associated impacts would be evaluated as part of that discretionary project.