Caltrans Devil’s Slide Realignment Project

The California Department of Transportation (Caltrans) invested in an expensive relocation of the Devil’s Slide segment of Highway 1, a coastal highway linking Half Moon Bay and San Francisco in San Mateo County. The segment was repeatedly closed due to damage from rockslides and erosion. The state decided to relocate the road at additional up-front expense (approximately $342 million) to avoid the long-term maintenance costs of rebuilding the road repeatedly in its existing location. Although not specifically implemented in response to climate change, this project provides an example of how realignment may present a cost effective strategy for adapting transportation assets in the face of mounting maintenance costs from repeated damage due to climate-related events. Landslides triggered by heavy rainfall events are anticipated to occur with greater frequency as climate change increases the number of extreme precipitation events in some regions. In a 2013 report, Caltrans recognized the Devil’s Slide project as an “unintentional” measure that it has taken to adapt to climate change.

Highway 1 is a scenic state highway that runs 123 miles along the central California coast. The highway is a curvy, cliff-hugging route that has been described as one of the most beautiful stretches of road in the world. In the segment of the road between Half Moon Bay and San Francisco the highway traverses a steep, unstable geological formation known as Devil’s Slide. This segment of the highway was subject to frequent closures from rockslides, the longest of which occurred in 1995 when the road was closed for 158 days causing drivers to take a 45-mile detour and costing almost $3 million to repair. 

Rather than continue to repair the highway in place, Caltrans invested in a realignment of the highway to bypass the geologically unstable slide area. The realignment included construction of a ¾ mile long tunnel and a bridge (“tunnel alternative”). The new highway segment is approximately 6,500 feet long. The project cost $439 million to construct and was funded with Federal Highway Administration (FHWA) Emergency Relief Funding. Construction was begun in January 2007 and completed in 2013.

Originally, an inland surface bypass was proposed as the relocation alternative for the Devil’s Slide segment of Highway 1; this bypass called for construction of a 6.8-mile alignment along Martini Creek (called the “Martini Creek alignment”). However, construction of the Martini Creek alignment was suspended after a court issued an injunction against the project in 1987 based upon environmental and aesthetic impacts to a state parkland through which the highway was going to be routed (Sierra Club v. U.S. Dep't of Transp., 664 F. Supp. 1324). The costs of mitigating the environmental impacts made this alternative as expensive or more expensive than the tunnel alternative. Caltrans had originally rejected the tunnel alternative as too expensive, but the alternative was further explored in a 1995 supplemental Environmental Impact Statement (EIS). In 1996, the tunnel alternative was considered reasonable and feasible, and FHWA identified the tunnel alternative as the preferred alternative in 2002.

In the EIS for the realignment project, Caltrans also studied and rejected several other alternatives. The no action alternative was rejected because of the high likelihood that periodic landslides would cause road closures and increasing maintenance costs. A marine disposal alternative, which entailed the deposit of fill to buttress the slope and relocate the road inland was rejected based upon cost, time, and inconsistency with federal laws governing marine sanctuaries. A dewatering alternative, involving horizontal drains and pumps, was rejected because of uncertain potential success and maintenance costs.

As part of the project, the old Highway 1 right of way was relinquished to San Mateo County. The county repurposed the old section of the highway to create a 1.3 mile hiking and bike trail, which will open up recreational opportunities and access to the coast, and will connect to the larger network of trails provided by the California Coastal Trail.


This Adaptation Clearinghouse entry was prepared with support from the Federal Highway Administration. This entry was last updated on October 31, 2015.


Publication Date: 2013

Related Organizations:

  • California Department of Transportation (Caltrans)


Resource Category:

Resource Types:

  • Best practice
  • Case study

States Affected:


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