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The Alto Tunnel will connect Mill Valley to Corte Madera, and southern Marin to the rest of Marin, enabling people of all ages and abilities to walk or bike safely, leave their cars at home, and enjoy the beauty of our communities for generations to come.

Frequently Asked Questions

Where is Alto Tunnel?

As shown in the map below, Alto Tunnel connects two popular multi-use pathways. The entrance from Corte Madera lies between Tunnel Road and Montecito Road, south of the Sandra Marker Trail. The Mill Valley portal is north of Vasco Court, past the dead end of the Mill Valley-Sausalito multi-use pathway.

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Why should the County reopen Alto Tunnel?

The vision for the North-South Greenway–a continuous pathway from Sausalito to Sonoma County along the former Northwestern Pacific Railroad right-of-way–dates back to the 1970s. Former rail corridors are great for people walking and bicycling because they are flat, separated from traffic, and conveniently run through the hearts of our communities.

Alto Tunnel is the lynchpin of the Greenway. Opening the tunnel will create a flat, safe, and direct community connector that will appeal to all ages and abilities. Camino Alto has steep climbs and descents with a lot of traffic and other drawbacks, so is not suitable for pedestrians, children, or individuals with disabilities, so Horse Hill is the best short-term option for now but the vision is to reopen Alto Tunnel.

How many people are expected to use the tunnel?

The Corte Madera to Mill Valley Bicycle and Pedestrian Corridor Study (2010) predicted that the tunnel connecting Corte Madera and Mill Valley would be used by about two to five thousand people daily (850,000 to 1.85 million annually).

How much would it cost to reopen Alto Tunnel?

The County’s recent structural investigation (2018) generated a $46.8m cost estimate to rehabilitate the tunnel. This estimate was based on the known costs from constructing the Cal Park Hill Tunnel in San Rafael in 2010, with inflation and a 20 percent contingency factored in. Adjacent pathway improvements are expected to bring the total cost to $52.6m.

Where will the money come from? Would funds to open Alto Tunnel take away from schools, parks, emergency services, or other essential needs?

No. The funds used to open Alto Tunnel would come from competitive federal, state, and regional grants devoted to active transportation and/or reducing greenhouse gas emissions associated with transportation. These funding opportunities are steadily increasing due to state policies aimed at improving our environment, health, mobility, and equity through transportation investments.

How will reopening the tunnel affect neighbors?

There are many neighbors who support our efforts and would love to have the option to travel to Mill Valley or Corte Madera by foot or bike in a few minutes. The supporters of efforts to reopen Alto Tunnel are committed to hearing and working to address any concerns that neighbors may have. This will work to ensure that the adjacent pathways 1) maintain the look and feel of a linear park and 2) are lined with berms, walls, fences, and/or landscaping to buffer any auditory or visual impacts on neighboring properties.

Case studies throughout the U.S. indicate that bicycle and pedestrian pathways are overwhelmingly safe, valued by communities, and lead to increased property values. That is especially true in Marin, where many people who live along multi-use pathways have added gates to their rear fences to enable easier access to those pathways.

What is the condition of Alto Tunnel today?

The County’s 2018 structural investigation of the tunnel found that the northern half of the tunnel remains mostly intact, while the southern half showed several areas with collapse.

What security features would be in place?

Like Cal Park Hill Tunnel in San Rafael, Alto Tunnel would be closed at night and include many safety features, including emergency phones, security cameras, cell phone reception, lighting, ventilation, and fire protection.

Who owns the property rights to Alto Tunnel and adjacent pathways?

The County’s property study (2016) concluded that the County holds all necessary rights to construct and operate a public pathway except for a side yard at the south portal and two sections retained by the railway, which previously offered to transfer its rights to the County. Upon completion, the side yard would remain as it is now, but the homes and properties above would be stabilized.

Are there any environmental considerations?

When the County moves forward with the project, environmental impacts and corresponding mitigation measures would be identified and made public through the environmental review process.

Reopening the tunnel will produce significant health and environmental benefits. The County’s 2010 corridor study projects that reopening the tunnel will result in nearly 2,000 fewer car trips and up to 15,600 fewer miles driven daily. The safe and level pathway will increase the number of people choosing to walk and bike to places, improving their health through exercise. Opening the tunnel will give people the option of a safe, car-free, and enjoyable way to walk and bike between northern and southern Marin.

How can I help?

We welcome your support! Go to our “Support” page to see how you can help re-open Alto Tunnel.