police say many times young joy riders are responsible. our carjacking offenders are increasingly younger and younger. we're trying to encourage our young people to make better decisions. >> reporter: another example of a rise in violent crime, the shootings of police officers from houston to d.c. in new york city the new mayor is promising more comes on streets and subways including the reinstatement of plainclothes officer units to step the flow of illegal firearms. >> we will not surrender our city to the violent view. >> reporter: police say one of the techniques is the bump and run technique where they initiate the carjacking by tapping on the vehicle. police say the main advice is to be aware of your surroundings especially in public places like gas stations. >> pretty scary. and for a second day in a row,