This elevated off-ramp in Martinez, California marks the site of the deadly Yuba City bus disaster.  On May 21, 1976, the Yuba City High School had a school bus chartering its a-capella choir from Yuba City to Miramonte High School in Orinda, California for a friendship day involving the choirs of both schools.  At about 10:55AM the driver Evan Prothero was taking the Marina Vista Avenue off-ramp southbound from I-680 when he noticed a low air pressure warning light come on.  He however didn't know that is what it was & mistook it as a low engine oil pressure signal instead.  As a result, he had lost all braking ability by the time the bus had exited the freeway & upon reaching this sharp corner it immediately struck the mounted bridge rail & left the elevated roadway, falling 21.6 vertical feet & landing on its roof, crushing it to the base of the window line.  Among the 52 on board, there were 29 fatalities, nearly all of them students ages 14 to 18.  An adult supervisor was also among the deceased.  The crash currently remains the second deadliest bus accident in American history. 

Since its inception, the crash currently remains the deadliest highway disaster ever investigated by the National Transportation Safety Board (NTSB).  The incident has led to more training & testing of school bus drivers & more regulated pre & post-trip inspections required by any CDL holder for any air-brake operating vehicle.  The off-ramp today has been reconfigured as of 2015 to lessen the sharpness of the curve.  Memorials to this particular tragedy have since been erected in both Martinez & Yuba City.

More info here: https://en.wikipedia.org/wiki/Yuba_City_bus_disaster