State Route 37 (SR 37) is a state highway in the U.S. state of California that runs along the northern shore of San Pablo Bay. It serves as a vital connection in the North Bay region of the San Francisco Bay Area, running from U.S. Route 101 in Novato, through northeastern Marin County, and the southern tips of both Sonoma and Solano Counties to Interstate 80 in Vallejo. Sonoma Raceway and Six Flags Discovery Kingdom are accessible from Highway 37. The highway has been proposed to be built to freeway standards since the early 1950s. However, the proposal was met with many economic and environmental obstacles, making the task all but impossible for much of the route. The route is plagued by flooding which can be exacerbated by levee breaks near Vallejo. Most of the highway crosses a marsh that is home to endangered salt marsh harvest mice. A section of highway was once known as "Blood Alley" for its high rate of fatal accidents. A concrete barrier built in the 1990s eliminated those fatal head-on collisions. Route description Intended to run from State Route 251, a highway that has not been constructed, SR 37 begins in Novato with a junction at U.S. Route 101 and heads northeast as a freeway for about a quarter mile before becoming a four-lane expressway. The route passes over the Petaluma River into Sonoma County before meeting the southern terminus of State Route 121 at a signal-controlled intersection near Sears Point and the Sonoma Raceway. The route continues as a divided two-lane expressway in a more easterly, then southeasterly, direction as it crosses Tolay Creek and proceeds through the Napa Sonoma Marsh at the northern edge of San Pablo Bay. Before the construction of the barrier in 1995, this portion of SR 37 was three lanes with the middle lane alternately serving as a passing lane for each direction. This stretch of highway was given the nickname of "Blood Alley" for its high-rate of fatal accidents. With the middle lane removed, accidents dropped dramatically. SR 37 becomes a four-lane freeway on Mare Island, approaching northern Vallejo. After it crosses over the Napa River Bridge, it continues as a freeway, overlapping the old highway alignment and passing north of the old road known as Marine World Parkway (due to its proximity to the Six Flags Discovery Kingdom, previously known as Marine World). SR 37 travels