A bicyclist was struck head-on by a pickup truck at the intersection of South Alamo and South Presa Street.
The accident happened south of downtown just before 7 pm Sunday evening.
The bicyclist was identified as Matt Shaw, who was taken to SAMMC with life-threatening injuries.
Police said Shaw was not wearing his helmet.
The driver was making a left turn in his Ford F-250 to go southbound on South Presa. Shaw was going northbound on South Alamo.
Police said the collision occurred in the middle of the intersection, and Shaw was thrown from his bike.
The driver and his girlfriend, who was in the truck, got out to help Shaw. They said he was bleeding from his head and nose and was barely conscious.
The driver's girlfriend said the couple had been drinking earlier in the day.
Police administered a field sobriety test to the driver, but he refused to take a breathalyzer test.
He was taken into custody so police could perform a blood test.
Investigators believe both the driver and Shaw had a green light and the accident may have been the result of a failure to yield.