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NTPP Project #1001

Last updated: October 21, 2016

This Alto Tunnel Study is an effort to collect information on the feasibility of reconstructing the Alto Tunnel for potential future bicycle and pedestrian use.  At this time, no decision has been made to reconstruct the tunnel.  The County is gathering technical information and details requested by decisionmakers to help them make an informed decision about whether to initiate this project or not.

For prior information on the 2010 Mill Valley to Corte Madera Bicycle and Pedestrian Corridor Study that includes the Alto Tunnel and two other routes, visit the project webpage.

The Alto Tunnel corridor is in the incorporated areas of Mill Valley and Corte Madera. The County of Marin is the lead agency on this multi-jurisdictional corridor study and works closely with staff from Mill Valley, Corte Madera, Transportation Authority of Marin, and County Parks & Open Space.

Geotechnical & Structural Review and Cost Estimate Update Phase - 10.21.16

This phase of the Study will seek further technical details about the tunnel to inform a refined cost estimate for potential reconstruction of the tunnel if it were to be used as a bicycle and pedestrian pathway. 

On April 28, 2016, a Request for Proposals was released for professional services for review of the geotechnical & structural condition of the tunnel and to update cost estimates for reconstruction of the tunnel for consideration of potential future use as a bicycle and pedestrian facility. These Questions & Answers for Proposers were prepared prior to submittal of proposals.  Proposals were due on May 26, 2016. 

On October 25, 2016, the Marin County Board of Supervisors will consider approval of a contract with McMillen Jacobs Associates for geotechnical and structural investigative services and to update previous cost estimates. It is anticipated that an informational meeting will be held after the holiday season and in early 2017 to communicate details and answer questions.  Following completion of the investigation and updated cost estimates, another meeting will be held to present the findings.  A more detailed timeline of upcoming milestones will be posted soon.

Upcoming Efforts

As with the MV-CM Bike/Ped Study, the County is the lead agency, in partnership and cooperation with the City of Mill Valley and the Town of Corte Madera.   The second phase, the geotechnical and structural analysis, will occur over the next year at which time a study report will be prepared discussing the outcomes of the overall study.


March 2013: Release RFP for land surveying and mapping

June 4, 2013:  BOS approval of contract for field surveying & mapping

April-mid 2013-15: Property research, evaluation and documentation

Mid 2013-15:  Field Surveys of Alto Tunnel corridor

Late 2015: Public Workshop on property research and mapping findings

April 28, 2016: Release RFP for geotechnical investigation

May 26, 2016: Proposals due

October 25, 2016: BOS approval of contract for geotechnical investigation

Early 2017 (tent.): Informational meeting on geotechnical phase process

Mid 2017 (tent.): Geotechnical phase field work

Late 2017 (tent.): Informational meeting on results of study & updated cost estimates


The bicycle and pedestrian connection between Corte Madera and Mill Valley was identified as one of the top priority projects in the county, based on public input in the 2006 Marin County Bicycle Plan Update public workshops. Currently, bicyclists use either Camino Alto or the U.S. 101 “Horse Hill” bike path via local neighborhood streets. As discussed in the 2010 Mill Valley to Corte Madera Bicycle and Pedestrian Corridor Study, it is challenging for cyclists and pedestrians alike to travel between the two communities due to a number of factors including steep topography, lack of facilities, and speed of vehicles. This is also part of the regional North/South bicycle route. 

Completed in 2010, the Mill Valley to Corte Madera Bicycle and Pedestrian Corridor Study evaluated improvements to three alternative routes between Mill Valley and Corte Madera, including (1) the Horse Hill/Alto Hill Pathway and connecting roadways, (2) reopening the Alto Tunnel for bike/ped use, and (3) the Camino Alto/Corte Madera Avenue on-street route.  Conceptual improvements were evaluated through an inclusive public process.  During this process, several additional issues were identified with the Alto Tunnel route that needed additional study.  These additional issues included the contingency levels (40%) on the project cost estimates, as well as property/right-of-way questions.

Learn more about the Alto Tunnel in the 2010 Mill Valley to Corte Madera Bicycle and Pedestrian Corridor Study project webpage.

The Marin County Board of Supervisors authorized additional federal Non-Motorized Transportation Pilot Program (NTPP) grant funds to refine cost estimates and investigate issues identified in the MV-CM Bike/Ped Study.  This additional research will further evaluate property and right-of-way issues, as well as collect additional geotechnical information on the condition of the tunnel itself.  This information will help narrow the construction cost estimate contingency level to achieve a higher level of confidence in how much it would cost if the tunnel were reconstructed for bicycle and pedestrian use. 

A Request for Proposals for land surveying and mapping was released on March 4, 2013 to provide current and accurate mapping of the Alto Tunnel corridor between Redwood Ave./Tamalpais Dr. in Corte Madera and Vasco Court in Mill Valley.  Concurrent with the land surveying and mapping efforts, the Marin County Real Estate Division conducted title research, documentation of ownership interests, and potential subsurface rights of property owners above the tunnel structure. This phase is complete.

Summary of the Alto Tunnel Corridor Right of Way Research - 3.25.16

 The first phase of the Alto Tunnel Study includes detailed research and analysis of property records, field surveys and mapping of the Alto Tunnel Corridor between Redwood Ave./Tamalpais Dr. in Corte Madera and Vasco Court in Mill Valley.  Below is a link to a draft narrative document summarizing the results of that property research, a Right of Way map showing the segments along the corridor that the County holds and those segments that others hold, a Boundary Map, and a Topographic Map of the Alto Tunnel corridor.  

The narrative summary and information posted to this webpage is for information purposes, as the County is only partway through the Study and it is not yet finalized.  It is provided early in the Study process as an opportunity for public review and comment, as this is a topic of interest to those residents close to the Alto Tunnel.  A Revised draft summary of the right-of-way research conducted by the County of Marin incorporated minor clarifications in response to public feedback and input.  Some of the clarifications include specifically identifying the private properties above the tunnel, as requested by some parcel owners.  Other clarifications are stylistic in nature and do not change the position or findings based upon the research conducted and information obtained or provided since the initial release of the draft research summary.  The information collected to date, as well as public input and additional information received during the course of the entirety of the Alto Tunnel Study process will help inform the refinement of cost estimates in the next phase of the Study.

Over 90 people attended an informational meeting on Tuesday November 17, 2015 at 5:30p, Mill Valley Community Center to hear about the initial right-of-way research along the Alto Tunnel corridor and to provide their feedback to County staff. View the presentation (pdf) from the November 17, 2015 informational meeting.  

Background and Technical Documents

How to Stay Informed

County staff will utilize the Interested Parties email lists for public notifications and information on the Alto Tunnel Study. If you are not on this list and would like to add your contact information for future correspondence; OR if you are on the list and do not want to receive Alto Tunnel Study information, please contact the County’s Project Manager, Ms. Carey Lando (see below).

A Public Workshop in Fall 2015 provided an opportunity to submit input and learn about the findings of the property research and land surveying.  As with the MV-CM Bike/Ped Study, County staff will maintain a fair and transparent process for providing information to all members of the public via website posting, notices and public meetings. An Informational Meeting in early 2017 will be held to communicated details and answer questions about the upcoming geotechnical and structural investigation phase of the study.

For questions about this study, please contact Ms. Carey Lando, Senior Project Planner, at or (415) 473-5078.