Vital North-South Greenway Link County of Marin Releases Alto Tunnel Cost Estimate

This Tuesday, the County of Marin  released the Alto Tunnel Investigation and Cost Estimate Update, signaling the completion of several years of information-gathering on the long-shuttered tunnel.

“MCBC is delighted that this day has finally come,” said Executive Director Jim Elias. “With detailed cost estimates in hand, we now know what it will take to close the last significant gap in the North-South Greenway.”

The former rail tunnel between Corte Madera and Mill Valley was built in 1884 and served rail operations until it was closed in 1971. It was first identified as part of the North-South Greenway – a multi-use pathway along the former Northwestern Pacific Railroad tracks from Sausalito to Novato – in the County’s 1975 Bicycle Plan.

Uncertainty about the tunnel’s condition and the cost to reopen it for bicycle and pedestrian access has long been an obstacle to moving the project forward.

The newly-released report was informed by laser scans taken at five locations in the tunnel. Based on the conditions observed, engineers produced cost estimates to rehabilitate ($46.8m) and permanently seal ($8.5m) the tunnel. The report does not include any information about possible funding sources for either effort.

The bicycle and pedestrian funding landscape is constantly evolving depending on state and federal policy direction. While much is uncertain at the federal level, California’s passage of Senate Bill 1 earlier in 2017 will result in $1b in funding for walking and biking infrastructure to be distributed via competitive grants over the next decade. The same cannot be said for the cost to permanently seal the tunnel, which would likely come directly out of the County’s wallet.

“For projects of this scale, Marin typically must look beyond County coffers to compete for bicycle and pedestrian funds,” said Policy and Planning Director Bjorn Griepenburg. “While cost will likely be a challenge to reopening Alto Tunnel, whether or not it receives funding will depend on how the project stacks up against others from around the region and state.”

MCBC will continue to assist Friends of Alto Tunnel towards reopening the tunnel.

“When reopened, residents of Marin will one day view the Alto Tunnel as the North-South Greenway’s equivalent to the Golden Gate Bridge,” said Elias.


The report released this week is the third and final study examining the tunnel’s feasibility as a bicycle and pedestrian connection. The Mill Valley to Corte Madera Study (2010) evaluated the three viable paths of travel for people walking and biking between the two cities and included conceptual cost estimates. Unsurprisingly, that study confirmed Alto Tunnel as the safest and most convenient potential route.

A property study (2015) found that the County holds all rights necessary to construct and operate a pathway leading up to both portals, but does not have the rights to three sections within the tunnel. The revised cost estimate released this week included right-of-way as a line item.

Next Steps

The County will host an informational meeting about the study at 6:30 PM Wednesday, September 27 at the Corte Madera Community Center, 498 Tamalpais Drive.


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