TO: Board of Supervisors
FROM: Steven Lederer, Director of Public Works
REPORT BY: Sydney Barclay, Associate Engineer
SUBJECT: Presentation regarding Edge line Rumble Strips and Amendment 2 to Agreement No. 250052B with Sanderson Bellecci, Inc. for the HSIP Cycle 11 “Silverado Trail Intersection Improvements” project, RDS 22-26.
RECOMMENDATION
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Receive a presentation on edge line rumble strips on Napa County roadways and provide direction; and approve Amendment No. 2 to Agreement No. 250052B with Sanderson Bellecci, Inc. for additional engineering design and support services for the “HSIP Cycle 11 Silverado Trail Intersection Improvements” project RDS 22-26. (Fiscal Impact: $267,532 Expense; SB-1 Fund; Budgeted; Discretionary)
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BACKGROUND
The Department of Public Works will provide a presentation to the Napa County Board of Supervisors regarding the advantages and disadvantages of using edge line rumble strips on County-maintained roadways, with a particular focus on their potential implementation along Silverado Trail. As directed by the Board, the “Bale, Big Ranch, Lodi, Silverado Trail, and West Zinfandel Paving, RDS 25-06” project, completed in the fall of 2025, included the installation of rumble strips as a pilot effort. The pilot installation has generated a range of feedback from nearby residents, businesses, and roadway users.
Public Works reviewed countywide collision data from fiscal year 2024-2025 and identified a total of 273 reported accidents in Napa County. Of these, 160 were single-vehicle collisions, with the majority classified as lane departure events. While some incidents involved external factors, such as animals crossing the roadway, the vast majority of accidents were attributed to driver inattention, distraction (commonly phone use), fatigue, or impairment (DUI, etc.). Of the 24 confirmed incidents in which drivers fell asleep, 11 occurred between 1:00 PM and 5:00 PM.
On Silverado Trail, the collision rate is approximately one accident per 40,000 vehicles, compared to one accident per 213,000 vehicles on Oak Knoll Avenue and one accident per 30,000 vehicles on Berryessa Knoxville Road. The collected traffic data indicates that collisions most frequently occur during the morning and afternoon commute periods, though incidents have been documented during all hours of the day. Fridays have the highest number of reported collisions, and while November has the highest monthly collision history, collisions occur throughout the year.
Certain locations along Silverado Trail experience a higher concentration of collisions (known as “hot spots”), and include the intersections of Oak Knoll Avenue, Zinfandel Lane, and the Strawberry Patch, which are being addressed by safety improvements with the Highway Safety Improvement Program (HSIP) Cycle 11 Intersection Improvement project. These hot spots were identified in Napa County’s Local Roadway Safety Plan (LRSP). A LRSP is required to apply for the HSIP grant funding and will be updated later this year to identify safety improvements and hot spots based on Napa County’s latest traffic incident data. Depending on the Board’s direction, rumble strips could be identified as a potential safety countermeasure within future LRSP updates and may be eligible for grant funding consideration.
Lane departure collisions along Silverado Trail generally occur at random locations rather than at a single reoccurring point and are often associated with moments of driver distraction or fatigue rather than with identifiable roadway deficiencies. While a history of collisions may suggest an elevated level of risk of another accident occurring at a particular location, the absence of prior incidents does not necessarily indicate that a location is free from a future risk.
Edge line rumble strips are widely recognized by the Federal Highway Administration and Caltrans as a safety countermeasure intended to reduce lane departure crashes. Studies indicate that edge line rumble strips can reduce lane departure collisions on two-lane rural roads by approximately 26 to 46 percent. Rumble strips consist of grooves milled into the pavement to create audible noise and vibration when driven over, to alert motorists that they are leaving the travel lane. This device is intended to reduce the severity and frequency of crashes associated with distracted, fatigued, or impaired driving. An additional potential benefit is that the audible warning may alert bicyclists when a vehicle encroaches into the shoulder area.
On Silverado Trail, the intended function of rumble strips would be to provide an audible and vibratory warning to drivers departing the traveled way, with a potential secondary benefit of increasing awareness between motorists and bicyclists when vehicles move into the shoulder or bike lane area.
While rumble strips have advantages, they also present potential drawbacks. The noise and vibration generated when vehicles drive over the strips may be disruptive to nearby residents, businesses, and visitors. Rumble strips also raise rideability concerns for bicyclists by reducing usable shoulder width and limiting maneuverability around debris or obstacles within the bike lane or roadway shoulder.
For the past decade, Napa County has evaluated and implemented a variety of safety measures along Silverado Trail. As part of the 2025 paving project, rumble strips were installed between Skellenger Lane and Zinfandel Lane as a pilot installation. Following their installation, some adjacent property owners expressed concerns regarding increased noise levels. One alternative to traditional rumble strips is the use of “mumble strips”, which are designed to reduce external noise impacts while still providing feedback to drivers. Another alternative is the installation of closely spaced reflective pavement markers.
In December 2023, Napa County installed reflective markers along the edge line striping at the intersection of Coombsville Road and First Avenue following a serious lane departure collision. Initial observations suggest that there has been a reduction in roadway departures at this location and the safety device has not posed a noise concern to nearby residences. County staff have also met with adjacent property owners and continue to evaluate the effects of rumble strips on Silverado Trail and the reflectors on Coombsville Road and First Avenue.
The Napa Valley Transportation Authority (NVTA) has prepared countywide bicycle plans and is currently developing the 2026 Countywide Active Transportation Plan, which includes recommendations for roadway safety improvements. County staff have met with NVTA and the Napa County Bicycle Coalition to discuss safety countermeasures such as rumble strips, mumble strips, bike buffers, and other treatments on County-maintained roadways.
The presentation to the Board will provide information on available safety data, design considerations, community impacts, alternative safety treatments, and guidance on the appropriate application of edge line rumble strips (or their alternatives) depending on nearby land uses and zoning in the unincorporated area of Napa County. Staff will also summarize public feedback received following the installation of approximately 3.5 miles of rumble strips along Silverado Trail between Skellenger Lane and Zinfandel Lane. The purpose of the presentation is to inform the Board, gather input from agencies and the community, and receive direction prior to making decisions that could affect future County policy and Capital Improvement Projects.
Aside from the presentation, this Board Item is also intended to approve Amendment No. 2 with the design consultant for the HSIP Cycle 11 Silverado Trail Intersection Improvements project.
On July 9, 2024, the Board of Supervisors approved Agreement 250052B with Sanderson Bellecci, Inc. to provide engineering, environmental, and construction support services for the HSIP Cycle 11 Silverado Trail Intersection Improvements project. Agreement No. 250052B was prepared for a maximum compensation of $191,183 for preliminary engineering services and was amended in March 2025 (PSA Amendment 1) to increase the maximum compensation to $262,269.
As part of the proposed improvements at Silverado Trail and Oak Knoll Avenue, additional engineering services are required to complete the final design. The project proposes to widen Silverado Trail north of Oak Knoll Avenue, on the west side, to construct a new southbound right-turn lane. This widening will increase the structural and traffic loading on an existing storm drain culvert, necessitating structural engineering analysis to confirm the culvert has adequate capacity to handle this loading. Public Works staff requests an amendment to increase the consultant’s scope of work, reallocate funding within the Agreement, and increase the consultant’s overall compensation by $5,263 for a total maximum compensation of $267,532 (PSA Amendment 2). Sufficient design contingency exists within the project budget and no additional funding is requested at this time.
The Plans and Specifications are at approximately 95 percent completion and are anticipated to be finalized in March 2026, whereupon, staff will return to the Board to request authorization to advertise the project, adopt the project’s CEQA documents, and proceed with requesting construction authorization from Caltrans.
Procedural Requirement:
1. Receive staff report
2. Public comment
3. Motion, second, discussion and vote
FISCAL IMPACT
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Is there a Fiscal Impact? |
Yes |
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Is it currently budgeted? |
Yes |
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Where is it budgeted? |
Project 24015 which is funded by SB-1 Non-Operating Special Revenue Fund (Fund 2440, Sub-Division 1220052). |
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Is it Mandatory or Discretionary? |
Discretionary |
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Discretionary Justification: |
The Board has the option to receive the presentation; an increased scope for the design consultant is necessary to finalize the project design and ensure structural stability of an underlying culvert at the Silverado Trail and Oak Knoll Avenue intersection. |
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Is the general fund affected? |
No |
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Future fiscal impact: |
Construction funds will be requested in Summer 2026. |
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Consequences if not approved: |
The County hired Consultant will not be able to provide a complete and structurally sound design for the proposed improvements at Silverado Trail and Oak Knoll Avenue. |
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Additional Information |
Strategic Initiative: Build Healthy, Connected Communities. |
ENVIRONMENTAL IMPACT
ENVIRONMENTAL DETERMINATION: Consideration and possible adoption of a Mitigated Negative Declaration. According to the Mitigated Negative Declaration, the proposed project would have, if mitigation
measures are not included, potentially significant environmental impacts in the following areas: the Strawberry
Patch and Oak Knoll Avenue. This project site is not on any of the lists of hazardous waste sites enumerated under Government code section 65962.5.