Closing the Gap from Christopher Ruedy.


The Alto Tunnel will connect Mill Valley to Corte Madera, and complete a greenway from southern to northern Marin, enabling people of all ages and abilities, whatever their purpose – commuting, transportation, exercise, recreation – to walk, bike , and roll safely and efficiently, enjoying 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 entrance is north of Vasco Court, past the dead end of the Mill Valley-Sausalito multi-use pathway.

Topographical map of Mill Valley and Corte Madera showing location of Alto Tunnel

Why should the County reopen Alto Tunnel?

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

Alto Tunnel can be the lynchpin of the Greenway. Opening the tunnel will create a flat, safe, and direct community connector serving the needs of riders and walkers of all ages and abilities. With its steep climbs and descents, lots of traffic and limited berms, Camino Alto is not a good choice for pedestrians, children, or individuals with limitations or disabilities. Horse Hill is the chosen short-term option for now, but the reopening Alto Tunnel will make the north-south greenway complete and fully accessible for all.

How many trips are expected to be made through the tunnel?

The Corte Madera to Mill Valley Bicycle and Pedestrian Corridor Study (2010) predicted that the Alto Tunnel would have bikers and walkers making two to five thousand trips daily. Riders who commute make repeated round trips throughout the year; other riders and walkers would be expected to travel through the tunnel on a regular basis, some daily, others several times a week. Still others would be occasional users. In total, a conservative estimate of annual trips through the tunnel is 500,000 to 800,000.

How much would it cost to reopen Alto Tunnel?

The County’s structural investigation (2018) generated a $46.8m cost estimate to rehabilitate the tunnel. The estimate was based on the known costs from constructing the Cal Park Hill Tunnel in San Rafael in 2010, including an inflation factor of 20 percent. Adjacent pathway improvements were expected to bring the total cost to $52.6m. It is assumed these cost estimates will need to be reviewed and revised given the length of time between the county’s study and a time when construction would occur.

Would funds to open the Alto Tunnel take away from schools, parks, emergency services, or other essential needs in Marin?

No. The funds needed to reopen Alto Tunnel should come from competitive federal, state, and regional grants devoted to active transportation goals 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?

Many, many neighbors support our continuing effort to see the Alto Tunnel restored. Having the option to travel back and forth between Mill Valley and Corte Madera by foot or bike in a few minutes has great appeal. As supporters of reopening the Alto Tunnel, we are committed to hearing and working to address any and all concerns that some neighbors have expressed.

Reopening the tunnel will include work to ensure that 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; the homes and properties above would gain by being 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 would result in nearly 2,000 fewer car trips and up to 15,600 fewer miles driven daily. No doubt these estimates would be higher today. The safe and level pathway will increase the number of people choosing to walk and bike instead of driving, contributing to their health through exercise. Opening the tunnel will give people the option of a safe, car-free, and enjoyable route to walk and bike between northern and southern Marin and beyond.

How can I help?

We welcome your support! Go to our “Get Involve” page to see how you can help re-open Alto Tunnel. We are asking people to sign a petition that will be delivered to the Marin County Board of Supervisors. Additionally, volunteers and donations are always welcome.