Waldo Grade

The Waldo Grade is a highway grade between the Golden Gate Bridge and Marin City along U.S. Route 101 and State Route 1. It is defined as the stretch of roadway between the Spencer Offramp and Marin City, within the city of Sausalito. This grade is traversed by a full freeway multi-lane highway facility. This portion of US 101/SR 1 is an important link in surface transportation connecting the city of San Francisco to Marin County and the North Bay. Nearby locations to the Waldo Grade include: the city of Sausalito, the U.S. Army Corps of Engineers Bay Model, The Marine Mammal Center and the Golden Gate National Recreation Area. It, and nearby Waldo Point along Richardson Bay between Sausalito and Mill Valley, is named after 1850s California politician William Waldo.

A tunnel officially known as the Robin Williams Tunnel is located at the highest elevation on US 101/SR 1 along the Waldo Grade. The first bore of the tunnel was completed in 1937 and the second in 1954. The archways at the south-facing ends of the bores were painted in rainbows by a Caltrans employee, Robert Halligan, and for this reason the tunnel is sometimes unofficially referred to as the Rainbow Tunnel.

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