Marin County Residents Debate Reopening Alto Tunnel for Cycling

Proponents argue it would provide a safer route, while opponents raise concerns over the high costs.

Published on Feb. 26, 2026

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The long-running debate over reopening the Alto Tunnel in Marin County resurfaced at a recent Mill Valley City Council meeting, with supporters arguing it would provide a safer cycling route and opponents raising concerns over the estimated $100 million price tag and engineering challenges.

Why it matters

The debate over the Alto Tunnel highlights the ongoing tensions between improving transportation infrastructure for cyclists and pedestrians versus the high costs and potential neighborhood impacts of such projects. The outcome could have significant implications for cycling and commuting in the Marin County region.

The details

Proponents, led by the Marin County Bicycle Coalition, urged the council to update and reaffirm its 2000 resolution supporting further study of the tunnel's reuse. They argued that reopening the tunnel would connect nearly 10 miles of multi-use path, offering a continuous, low-stress route separate from car traffic. Supporters emphasized the safety benefits for families and commuters currently using the steep and heavily trafficked Camino Alto and Horse Hill Lane routes. Opponents, including a local homeowners association leader, argued the project would require a full reconstruction rather than a simple reopening, citing tunnel collapse and homes built above the alignment. They suggested total costs could reach $100 million or more once inflation, easements, and related work are considered, and proposed the lower-cost Horse Hill route as an alternative.

  • The Mill Valley City Council meeting where the tunnel debate resurfaced was held on February 26, 2026.
  • The council's previous resolution supporting further study of the tunnel's reuse was passed in 2000, over a quarter-century ago.

The players

Warren Wells

Policy and planning director for the Marin County Bicycle Coalition, who asked the council to update and reaffirm its 2000 resolution supporting further study of the tunnel's reuse.

John Palmer

A Scott Valley resident and homeowners association leader, who argued the project would require a full reconstruction rather than a simple reopening, and suggested total costs could reach $100 million or more.

Stephen Burke

A Mill Valley city councilmember who noted the council would need to balance competing priorities and limited staff capacity when considering if and when to bring a tunnel resolution back for formal discussion.

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What they’re saying

“We are here to ask that you update that quarter‑century‑old council resolution to reflect the work that has been done to date, and renew the request that the outstanding issues be evaluated by the county.”

— Warren Wells, Policy and planning director, Marin County Bicycle Coalition (Patch.com)

“There are alternatives which could be improved, pointing to the Horse Hill route as a lower‑cost option.”

— John Palmer, Scott Valley resident and homeowners association leader (Patch.com)

What’s next

The Mill Valley City Council will need to decide whether and when to bring a formal resolution on the Alto Tunnel project back for discussion, balancing competing priorities and limited staff capacity.

The takeaway

The debate over the Alto Tunnel highlights the ongoing challenges of improving cycling and pedestrian infrastructure in Marin County, as local officials must weigh the potential safety and connectivity benefits against the high costs and potential neighborhood impacts of such projects.