San Francisco police are investigating an unknown Uber driver for robbing a passenger and keeping him from leaving the car, a police spokesperson said.The passenger had requested a lift through the ridesharing app, but when the black sedan reached the drop-off location around Embarcadero and Brannan Street at about 1 a.m. Saturday, he was at the driver’s mercy and stuck in the car, according to the police report.The victim was eventually able to get out of the vehicle and run from the driver, who chased the passenger and took his cell phone and debit card. The suspect took off in an unknown direction while the Uber user sustained a dislocated knee.Uber has been criticized for its screening of drivers, leading to inconsistent and potentially dangerous rides — which often occur at night. In San Francisco’s most extreme case, Roberto Chicas accused UberX driver Patrick Karajah of hitting the passenger on the side of the head with a hammer, according to court documents. A verbal dispute ensued when Chicas and two friends noticed Karajah had taken a different route and ended with Chicas bleeding and unconscious, with his left eye swollen shut.Karajah pleaded not guilty of assault with a deadly weapon and battery with serious bodily injury.In California, Assemblyman Adrin Nazarian introduced Bill 24 last month requiring rideshare drivers to undergo more vigorous hurdles including a fingerprint background check. A similar bill introduced last year was defeated.