I stupidly did it a once in Oakland a few decades ago as a one off risk, got hand job for $50 from good looking, busty white girl (I think was young unwed mother,) who took her top off, just parked in large parking lot a few blocks away, locked car doors. It was clearly, a quick way to make cash.
But yeah, it's pretty risky in some neighborhoods. With hindsite, I think risk wasn't SWs or pimps, but other low lifes who recognize if a monger is getting his knob polished in his car, and then mugging parked car by breaking window, holding knife or gun to monger who they figure is cash rich and unlikely to go to police.