Waldo Grade, Highway grade in Sausalito, California, United States

Waldo Grade is a multi-lane freeway grade on US 101 and State Route 1, running between the Spencer Offramp and Marin City within Sausalito. At its highest point the road passes through the Robin Williams Tunnel, whose two bores were completed in 1937 and 1954, with the Waldo Sidehill Viaducts alongside. The south-facing tunnel archways are painted in rainbow colours.

The Waldo Grade is a multi-lane freeway section of US 101 and State Route 1 between the Spencer Offramp and Marin City, within Sausalito. Its first tunnel bore was completed in 1937 and the second in 1954, together with the Waldo Sidehill Viaducts. The fill material along the grade had been stable since highway construction in 1953.

The grade and nearby Waldo Point on Richardson Bay are named after William Waldo, a California politician of the 1850s. The tunnel's south-facing archways were painted in rainbow colours after lobbying by Caltrans employee Robert Halligan, so drivers call it the Rainbow Tunnel. In 2016 the bore was officially renamed for actor Robin Williams, a longtime Marin County resident.

Southbound drivers get their first view of San Francisco and the Golden Gate Bridge on exiting the tunnel's southbound bore, since hills hide both from the northern approach. The grade runs between the Spencer Offramp and Marin City, within Sausalito. After the January 1982 landslides, northbound lanes were closed twelve days while the road was reinforced with vertical pilings.

The January 1982 landslides destroyed a house at 85 Crescent Avenue, killing resident Sally Baum, and 300 residents were evacuated to a Red Cross shelter. A crack in the roadway led Caltrans to close all northbound lanes for twelve days while the road was reinforced with vertical pilings, cutting Marin County off from the San Francisco Peninsula.