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Re-Opening the Alto Tunnel

Alto Tunnel - Linking Mill Valley and Corte Madera


Photo-simulation by Doug Nelson - RHAA

Updated: November 16, 2015

Alto Tunnel Property Study Released!

MCBC is thrilled to share that the long-awaited Draft Alto Tunnel Corridor Right of Way Study is now available. The study, prepared by the County of Marin, includes detailed research and analysis of property records, field surveys and mapping of the Alto Tunnel Corridor between Corte Madera and Mill Valley. The draft study outlines the findings of the property boundary and easement-related research toward resolving debates about tunnel easement ownership and helps refine the accuracy of cost estimates for reopening the Alto Tunnel.

If you support reopening the Alto Tunnel for safe bicycling and walking between Mill Valley and Corte Madera and closing this significant gap in Marin's North-South Greenway, now is the time to get informed and get involved! Please attend the November 17, 2015 meeting to learn more.

WHAT: Draft Alto Tunnel Corridor Right-of-Way Study Meeting
WHEN: Tuesday, Nov 17, 2015 at 5:30 pm
WHERE: Mill Valley Community Center, 180 Camino Alto, Mill Valley

The next phase of the Alto Tunnel Study, the geotechnical evaluation phase, will begin after the public input period for the Right of Way Study ends on December 1, 2015. The geotechnical phase will seek further technical details about the tunnel to inform a refined cost estimate for potential reconstruction of the tunnel as a bicycle and pedestrian pathway. Once this phase is completed, additional opportunities for public input and review will be held.

In addition to attending next Tuesday's November 17 meeting, here are two more ways to show your support now for reopening the Alto Tunnel:

Alto Tunnel Property Study Moves Forward

On June 4th, 2013, the County Board of Supervisors authorized an agreement with BKF Engineers for a study of properties that surround the Alto Tunnel.  The property study is the first of two Alto Tunnel studies that on August 23, 2011, the Marin County Board of Supervisors voted to allocate $600,000 of Nonmotorized Transportation Pilot Program funds to conduct. The second is a geotechnical study.

The $68,920 property study will refine cost estimates and investigate property boundary and easement-related issues identified in the 2010 Mill Valley to Corte Madera Bicycle and Pedestrian Study. MCBC is excited to see this study moving forward as it will help answer many of the most controversial questions regarding reconstructing the Alto Tunnel. 

After this study is completed and findings presented to the public, work will begin on a geotechnical study and analysis of the tunnel beginning later this year (2013). This second study will involve getting into the tunnel and will provide much needed information that will help provide more accurate cost estimates for tunnel reconstruction, helping to move the project forward.

These studies will increase the accuracy of cost estimates for rebuilding the Alto Tunnel and will help to resolve debates about tunnel easement ownership. Work on both studies will be completed in 2013/2014.

Please Support Re-Opening the Alto Tunnel: 
Sign the Re-Open Alto Tunnel Petition

As of June 6th, 2013, the Re-Open Alto Tunnel Petition has over 2500 signatures most of which you can view here

The Marin County Bicycle Coalition is seeking your support to re-open the Alto Tunnel. Re-opening the Alto Tunnel will close the gap that currently exists for safe bicycle/pedestrian travel between Mill Valley and Corte Madera. Our online petition describes reasons why the tunnel should be re-opened.  Signing the petition takes about one minute of your time. Thank you for your support!

There has been a significant amount of misinformation circulated about reopening the Alto Tunnel.  Many members of our community are forming opinions about the tunnel based upon rumor and hearsay. Click the following link to read MCBC's response to rumors and common misconceptions about the Alto Tunnel.

Alto Tunnel Study Funded as part of $8.8 Million Pilot Program Funding Allocation

On Tuesday, August 23, 2011, the Marin County Board of Supervisors voted to allocate $8.8 million for bicycle/pedestrian projects throughout Marin. The Marin County Bicycle Coalition fully endorsed the list of funded projects that were proposed by the County Department of Public Works, which were unanimously approved by the Board of Supervisors. The $8.8M in funds that were allocated are federal transportation bill extension funds related to the $25 million Nonmotorized Transportation Pilot Program, which MCBC's former Director Deb Hubsmith played a key role in bringing to Marin in 2005.

While MCBC is celebrating the allocation of all of these funds, we are particularly excited about the allocation of $600,000 of funds for studying the Alto Tunnel.   Funds will be used to conduct a geotechnical study to more accurately assess the tunnel's condition, which will result in more accurate cost estimates for building the tunnel.  In the June 2010 Mill Valley to Corte Madera Bicycle and Pedestrian Corridor Study, tunnel reconstruction estimates contained high contingencies that drove the tunnel estimates higher than they would otherwise be with up-to-date geotechnical information. This study will help resolve cost estimate issues.

Funds will also be used to conduct a property study, including establishing easement ownership for properties above the tunnel.  This too will help increase the accuracy of cost estimates for rebuilding the Alto Tunnel and help resolve debates about tunnel easement ownership.

Board of Supes accept MV to CM Study

Public overwhelmingly supports Alto Tunnel

On Tuesday September 21st, 2010 at a Marin County Board of Supervisors hearing, more than two dozen Marin residents spoke in favor of reconstructing the Alto Tunnel between Mill Valley and Corte Madera for bicycle and pedestrian use.

The meeting had the primary purpose of presenting to the Board the findings of the $225,000 Mill Valley to Corte Madera Bicycle and Pedestrian Corridor Study, which analyzed three routes between Mill Valley and Corte Madera: Camino Alto, Horse Hill and the Alto Tunnel. At the meeting, the Board voted to accept the Study as complete.

The final draft of the County of Marin Study indicates that as many as 1,850,000 people could use the Alto Tunnel annually, providing Marin County residents with a safe, separate (from automobiles), and flat route between the Mill Valley and Corte Madera. Public comments at the Board of Supervisors meeting ranged from how the tunnel would increase physical activity, thereby reducing childhood obesity, to how it would provide emergency egress in the event of wildfire. MCBC and several residents also mentioned the need for additional study of properties above the Alto Tunnel to resolve property questions, and to conduct geotechnical work inside the tunnel to develop more accurate cost estimates and reduced contingency margins.

The Marin County Bicycle Coalition applauds the Marin County Department of Public Works for their great work on this report and the Board of Supervisors for accepting it on Tuesday. We look forward to continuing to work together to improve the bicycle and pedestrian routes between Mill Valley and Corte Madera.

The County of Marin Releases its Final Draft of the Mill Valley to Corte Madera Bicycle and Pedestrian Corridor Study

The County of Marin has released its final draft of the Mill Valley to Corte Madera Bicycle and Pedestrian Corridor Study which was completed by consultants Alta/LandPeople at a cost of $225,000.  The final draft of this Study contains significant improvements over the previous draft  including a new higher-end estimate of tunnel use projections.  The new estimate, which is based upon current increasing bicycle use trends show that the tunnel may be used by as many as 1.85 million users per year.

The County of Marin will be holding a hearing on this Study on September 21st, 2010 at the County Board of Supervisors' Chambers.  For more information about this meeting, visit

MCBC would like to see further study be completed on the Alto Tunnel that includes geotechnical work and the resolution of property issues above the tunnel, both of which will help provide for more accurate cost estimates for re-building the tunnel.  Currently the cost estimates have wide error margins or contingencies because the condition of the tunnel is widely unknown. 

The Marin Countywide Plan, adopted in 2007 includes a focus on sustainability, climate protection and health, and contains a goal for 20% of trips in the County to be made by walking and bicycling by the year 2020. In 2006, Marin’s bicycle and pedestrian mode share was already at 13.6%, and a fall 2009 County study showed that bicycling has more than doubled in Marin over the past decade. As transportation represents 62% of greenhouse gas emissions in Marin, and state law now requires the region to cut emissions, there is increasing attention on reducing our ecological footprint. We can make a dent in reducing carbon emissions by building facilities that will get a maximum number of people out of cars. The Alto Tunnel can help the County meet its stated goal, while providing a wide range of health and environmental benefits to Marin County residents.  MCBC continues to work closely with the County of Marin to make Marin a model bicycle and pedestrian community for the nation. This will improve public health, reduce traffic, increase safety, decrease greenhouse gas emissions, and benefit communities, residents, and the economy throughout Marin.

Where is the Alto Tunnel and what is its history?

The Alto Tunnel was built in 1884 and is located in Marin County along the former Northwestern Pacific Railroad right-of-way, connecting Mill Valley and Corte Madera. The tunnel is 16-feet wide and 20-feet high, approximately 2200 feet (0.4 miles) long, and served as a single-track rail tunnel for nearly ninety years. In 1958, there was a substantial upgrade to the northern portal, which remains intact and in good condition today. The tunnel remained open until 1971, when substantial bulkheads were added at each portal to prevent entry. In 1975, a plug extending approximately 125-feet was added near the north end of the tunnel and in 1981, there was a collapse at the southern portal. The southern portal area was stabilized in 1982 with gravel and earth fill, which remains today. The middle 1600-feet of the tunnel remains sealed off.

Read the MCBC’s letter to the County, 3/31/09

On Tuesday, November 18, 2008, the Marin County Board of Supervisors approved of a $225,000 contract with Land People to conduct the Mill Valley to Corte Madera Bike and Pedestrian Corridor Study, which is part of Marin County’s Nonmotorized Transportation Pilot Program (NTPP). The study will be focused on three corridors between Mill Valley and Corte Madera: Camino Alto/Corte Madera Avenue, Alto Tunnel, and Highway 101 via the “Horse Hill” path.

The contract is providing for corridor planning and geotechnical feasibility work, public outreach, bicycle and pedestrian use projections, and preliminary design work. Current public outreach includes (or will include) a project website, fact sheets, an e-newsletter, press releases, presentations at public meetings, informational meetings with neighborhood groups, and an online survey. The Alto Tunnel analysis will build on lessons learned from the Cal Park Tunnel Multi-Use Path project.

The first public meeting took place on March 4, 2009 as part of this study.  The County solicited public comment regarding existing conditions, concerns and opportunities for each of the 3 routes.  The Marin County Bicycle Coalition submitted comments that focused on, amongst other things, the criteria for evaluating each of the three routes.  

The completed report will indicate the additional costs necessary to move to the next level of accuracy for the Alto Tunnel rehabilitation as a subsequent phase.

For more information about this study (timeline and scoping documents) along with previous scoping documents, including the 2001 Alto Tunnel Scoping Study prepared by Quincy Engineering for the Marin County Department of Public Works, please visit

The Marin County Bicycle Coalition thanks the Board of Supervisors for moving forward with the Mill Valley – Corte Madera Gap Closure study; this project is an important component of the North-South Greenway, a plan to create a flat and direct bikeway separated from cars, parallel to Highway 101, from Sausalito through Sonoma County.

Frequently Asked Questions about Re-Opening the Alto Tunnel


How many people are projected to use the Alto Tunnel on a daily basis?

Alto TunnelThe Alto tunnel would close the gap in the North South Greenway, providing a level, separated, non-motorized connection from Sausalito through Mill Valley to Corte Madera, Larkspur the upper Ross Valley, and to San Rafael. The Final Version of the  Mill Valley to Corte Madera Bicycle and Pedestrian Corridor Study  provided estimates for tunnel use.  These values range from a stated conservative estimate of 850,000 users to 1.85 million users per year. 

Why would the County of Marin want to restore Alto Tunnel?

The County of Marin owns the property on either side of the tunnel, and the railroad right-of-way has always been slated for use as a transportation corridor. Since the 1970s, the County has had plans for a North-South Greenway which would run along the Northwestern Pacific Railroad right-of-way, spanning Marin County from North to South, providing access to transit centers, businesses, schools, and town centers. There is currently a well-used multi-use pathway that extends for three miles, from Sausalito and through Mill Valley, leading right up to the south portal of Alto Tunnel. Another pathway continues north from the north portal of Alto Tunnel, through Corte Madera and Larkspur, approaching the Larkspur trestle. Here are some benefits of "closing the gap" in the North-South Greenway through opening the Alto Tunnel for bicycle and pedestrian use:

  • Improved Safety: Currently, bicyclists and pedestrians have two choices for traveling between Mill Valley and Corte Madera, neither of which are appealing. They can go out of their way to the "Horse Hill" (Alto Hill)/Highway 101 Freeway path (which includes 200 total feet of climbing and carbon monoxide fumes from the freeway traffic), or travel over Camino Alto, which is a 320 foot climb along a busy, narrow, winding road. Police statistics from Corte Madera indicate that there have been numerous injuries of non-motorized users along Camino Alto over the past ten years.
  • Decreased Traffic Congestion: Opening Alto Tunnel would create a straight, flat pathway that would be accessible to people of all ages and physical abilities. People would be able to commute to San Francisco, access the Manzanita Transit station, shop at businesses on either side of the hill, attend soccer practice and other after-school activities, go to the Mill Valley Community Center and the pool, and to commute to the College of Marin. Many auto trips could easily be replaced with bicycle or pedestrian trips once the tunnel is open.
  • A Route for People in Wheelchairs: Opening the Alto Tunnel would provide a socially-equitable transportation option for people in wheelchairs; there are two residential communities that support disabled people in the vicinity of the Alto Tunnel in Mill Valley.
  • Physical Health Benefits: The U.S. Centers for Disease Control and Prevention estimate that 300,000 Americans die each year due to complications associated with obesity and sedentary lifestyles, and that 67% of US adults are overweight or obese. The CDC recommends 30 minutes of physical exercise, five times a week, for adults and states that replacing trips by automobile with walking or cycling is one of the best ways the get this exercise.
  • Environmental Health Benefits: The useful route provided by the tunnel will replace motor-vehicle trips, reducing air and water pollution.
  • Emergency egress benefits of Alto Tunnel: There has been much attention placed on the slogan “Get Ready Marin” which urges residents to be prepared in the event of a disaster. The tunnel would provide an important emergency egress should there be a fire or other emergency.

What have been the experiences when other bicycle and pedestrian tunnels have been opened throughout the United States?

Several years ago, the County of Marin commissioned the Rails-to-Trails Conservancy to do a study of tunnels, which have been opened throughout the United States for use by bicyclists and pedestrians. The report concludes that tunnels are overwhelmingly safe, that they are valued by communities, and that there is a trend throughout the United States to open more tunnels to non-motorized use. "Tunnels on Trails, A Study of 78 Tunnels on 36 Trails in the United States" covers issues related to: safety, liability, costs of tunnel restoration, construction, maintenance, and the impacts of tunnels on the existing transportation network and surrounding communities. The study also found that property values often increased, and that while all neighborhoods had concerns before the tunnels were opened, some of the most vocal opponents of the tunnels came to be the most frequent users and supporters of the tunnels once they were opened. Click here to read the RTC study on-line in PDF format.

What is the condition of the Alto Tunnel today?

Map of Alto TunnelThe northern 170-feet was inspected in June, 2001 and determined to be in good structural condition. The exact condition of the middle 1600-feet of the tunnel is unknown at this time; however, further investigations are being proposed to better determine the condition in this zone. The existing redwood timbers may be in various stages of decay, depending on the amount of moisture in the sealed portion of the tunnel. Regardless of the existing conditions, the existing redwood timber support system will have to be replaced and augmented with a new concrete liner for both structural safety, aesthetics, and to eliminate any fire danger. The Southern 350-feet was backfilled with gravel in 1982 and will have to be excavated and supported with a new concrete structure.  MCBC is requesting that the County of Marin do further goetechnical study on the tunnel to learn more about the condition of the tunnel so more accurate cost estimates can be determined.

How much would it cost to re-open the Alto Tunnel and make it a part of the North-South Greenway?

Cost estimates in the final draft of the Mill Valley to Corte Madera Bicycle and Pedestrian Corridor Study range from $40 to $50 million but contain several sets of contingencies that range from 15% to 20%.  We believe that these estimates will be reduced substantially when engineers have an opportunity to get inside the Alto Tunnel and evaluate the tunnel's condition. 

Where would the money come from to open the Alto Tunnel?

There are a variety of potential transportation funding sources for the project. In addition, once a commitment is made to making a capital project a reality, new funding sources often emerge. The 1100 foot long Cal Park Hill Tunnel is being funded by the Bicycle Transportation Account, Transportation Funds for Clean Air Program, Transportation Enhancements, and Regional Measure 2, through a collaborative effort by the County of Marin and Sonoma Marin Area Rail Transit (SMART). When assessing the costs for Alto Tunnel, it is important to also look at the costs for other transportation projects. The HOV gap closure project which added one traffic lane along Highway 101 for 4 1/2 miles from Terra Linda to Larkspur cost more than $150 million. It will cost $800 million to widen the Highway one lane in each direction from Novato to Petaluma.

Would funds to open Alto Tunnel take away from schools, parks, emergency services, or other projects?

No. The funds that would be used to open the Alto Tunnel would be transportation project funds; they would not take away from schools, parks, emergency services, etc.

How would security and maintenance be handled?

Once the Alto Tunnel is opened, there will be annual costs to maintain and operate this facility. Some security measures that have been used in other tunnels and are planned for Cal Park Hill Tunnel include: security cameras, police patrols, and lighting. These are likely the types of features that we would see if Alto Tunnel were reopened. Because the portals to Alto Tunnel enter and exit in Mill Valley and Corte Madera, cooperation and coordination among the towns and the County of Marin would be essential.

How will opening Alto Tunnel affect the neighbors?

There are some neighbors near Alto Tunnel who enthusiastically want to see the tunnel re-opened, and some who are against the project and have major concerns. It will be important to open up more of a dialog with the communities immediately near the portal. There are many ways to minimize any possible negative impacts that an open the tunnel may have. Landscaping can create a visual barrier to the pathway, and sound mitigation can help to reduce any errant noise. Signs can be posted urging tunnel users to please be quiet and respect the neighbors. In addition, as with any transportation or development project, neighbors’ concerns must be weighed against the benefits of the project to the community at large.

Are there any environmental considerations?

All transportation projects have environmental considerations; the Alto Tunnel project would need to go through review under CEQA, the California Environmental Quality Act. If the County of Marin were to determine that they were ready to move forward with opening the Alto Tunnel, the appropriate departments within the County of Marin would determine the type of environmental review necessary. Because this project is a "replacement or reconstruction of existing structures or facilities" it has a different status of review than an entirely new transportation project.

Overall, opening the Alto Tunnel would produce significant health and environmental benefits for Marin County. People's health would benefit from increased walking/jogging/cycling, and environmental benefits would be accrued from motor vehicle trip reduction. The existence of a flat, straight, car-free route would attract people who might walk or bicycle today, but are dissuaded due to the hills and current levels of automobile traffic on surface streets.

How can I help?

For more information on how you can help get the Alto Tunnel opened, please contact Alisha Oloughlin at

Last updated: November 16, 2015