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The Waldo Grade is a highway grade between the Golden Gate Bridge and Marin City on U.S. Route 101 in California. 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. The highest elevation on this freeway section is at the Waldo Tunnel. This portion of U.S. Route 101 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.
Waldo Tunnel is the unofficial[why?] name of a tunnel on U.S. Route 101 between the Golden Gate Bridge and Sausalito. It is named after Waldo Point along Richardson Bay between Sausalito and Mill Valley. Waldo Point is named after William Waldo, a California politician in the 1850s who became well known for his relief efforts on starving immigrants coming West who got stranded in the Sierra Nevada Mountains on the Nevada side during a winter snow storm. He later ran unsuccessfully as a Whig candidate for the governor of California in 1853.
The first bore of the tunnel was completed in 1937 and the second in 1954. The archways at the ends of the bores were painted in rainbows by a Caltrans employee, Robert Halligan, and for this reason the tunnel is occasionally referred to as the "Rainbow Tunnel".
As San Francisco and the Golden Gate Bridge are hidden from the northern approach of Route 101 by hills, it is likely that some portion of visitors receive their first view of the city and the bridge upon exiting the tunnel's southbound bore.
On January 4–5, 1982, U.S. Highway 101 was completely closed for 24 hours as a result of two landslides on the Waldo Grade caused by a severe storm in the San Francisco Bay area, and partially closed for nearly two weeks.
The first landslide was on January 4, with rock, mud and trees falling onto the highway blocking the southbound lanes and two of the northbound lanes.
A second debris avalanche began about 50 feet (15 m) below Highway 101 at about 9:35pm on January 5, in fill material that had been stable since highway construction in 1953. The slide first carried away a house on Sausalito Boulevard, and then destroyed a house below it at 85 Crescent Avenue, killing resident Sally Baum. 300 residents were evacuated to a Red Cross shelter.
A crack developed in the roadway and, concerned that the highway might fail, CalTrans engineers closed all northbound lanes for twelve days while the road was reinforced with vertical pilings. The closure cut off Marin County from the San Francisco Peninsula. Thousands of Marin County residents were stranded in San Francisco who could not figure out how to get to Marin by using the The Richmond-San Rafael Bridge I-580 or the bus lines that serve Marin from the East Bay. Some of the earth from the landslide was later transported to Mill Valley to create a building pad for the Shelter Point office complex.
After the suicide of Robin Williams in 2014, a petition was started on Change.org to rename the Waldo Tunnel as the Robin Williams Tunnel.  Assemblyman Marc Levine stated he would explore the issue further after consulting with Robin's family. 
The tunnel is featured in the Clint Eastwood film Dirty Harry and the Humphrey Bogart film Dark Passage. The honking of horns in the tunnel, often done deliberately for the sake of hearing the echoes, was the inspiration for harmonicist Bruce "Creeper" Kurnow's composition Honk If You Love Harmonica.
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