The accident left the taxi driver seriously injured, and two sheriff's deputies who helped pull the victims out of the car were hospitalized for smoke inhalation.
The couple were on their way from San Francisco International Airport to the Mark Hopkins San Francisco hotel when the cab's brakes apparently failed, CHP Officer Shawn Chase said.
The crash was reported at 10:57 a.m. at the Mariposa Street off-ramp from northbound I-280 in the city's Potrero Hill neighborhood.
The woman was pronounced dead at San Francisco General Hospital shortly after the crash, and the man died at about 4:20 p.m. today, hospital spokeswoman Rachael Kagan said. The driver was seriously injured but left the hospital this afternoon.
Chase said the taxi driver reported that he had smelled smoke as he was driving on the highway.
He continued on and took the off-ramp, but when he tried to hit the brakes, they failed, Chase said. The driver said he also tried unsuccessfully to use the emergency parking brake.
The taxi was traveling at about 40 mph when it smashed into a highway support beam and the taxi caught fire, Chase said. It then hit a steel mesh fence.
Sheriff's spokeswoman Susan Fahey said three sheriff's deputies in two separate vehicles were driving north on the highway shortly before the crash.
They saw smoke coming from the cab on the highway and watched as the taxi exited and crashed, Fahey said. They headed toward the crash site to assist.
Chase said the taxi driver was able to exit the car on his own, and two deputies pulled the two passengers out of the burning car. Two of the deputies suffered smoke inhalation and were taken to St. Francis Memorial Hospital.
Fahey identified the three male deputies as Z. Ralleta, with the department since March 2007; R. Rood, with the department since September 1998; and C. Sheriff, with the department since August 2001.
Chase said the taxi was Dodge Intrepid from the Desoto Cab Co. and was registered in San Francisco. A spokeswoman for the company did not return calls seeking comment today.