They're an easier target when they're on the sidewalk.
A few weeks ago, there was a one-day furor in Detroit over the shooting/squishing death of a perp. Someone had been going around beating women and girls with a piece of rebar for several weeks, but the police had no lead.
The scumbag was attacking a couple of girls on a street corner, when a silver Chevy stopped, the passenger leaned out the window, and emptied a number of 9mm rounds into the perp. Then they backed up, drove over the curb, and drove over the guy. A woman in a car behind them saw everything, and also jumped the curb and ran over the guy, just to be sure. Both cars then drove away.
The media covered the story in some detail the next day, although the dozen or so witnesses could not agree on a description of the rescuers, or the color(s) of the car(s). One witness speculated the unfortunate bad guy had actually been struck be a meteorite.
The police said they were close to making an arrest, but were giving the good samaritans one day to turn themselves in. Since then (about two weeks ago), not one word has been said. I really don't thing the Detroit police (as opposed to the NYPD in the movie "Death Wish") are busting their butts on this case.