I spit a hard candy I happened to be sucking on at a cab, driver's side window which made a loud noise and seemed to get his attention. Hard bang with fist on roof right over driver's head gets noticed too. I've done worse but it's dangerous to hit a beloved automobile with your lock. I thought I could escape a van I hit w my lock but when he kept chasing after I went the wrong way on one way street, I gave up, had to apologize explaining that he had frightened me and I reacted out of fear (basically asking him not to kill me in different words).