Doesn't always work, either here or on the continent. I've know of (cycle) riders who've been killed or injured by such muppets - including one World Champion who was killed the year after winning the title at Mallory Park back in the 70s.
Not so much of a problem at our local races. You tend to only get wives and girlfirends (sometimes both), relations, plus perhaps a man walking his dog as the main spectators. If it is a local championship we might get, ooh let's see, a hundred people or so round the course and they tend not to be numbnutts but are fully aware of the high speed cycle racing can involve.
Joking apart though, the biggest hazard for the riders and the marshalls are the impatient bast*rds in cars who can't tolerate the maybe five minutes delay to their journey that a bike race might cause, and cut in and out of the bunch in the race, and can cut the m'bike marshall up as well.
They are obviously fully qualified Arseholes.
Never store your vaseline next to your superglue.
The spout will fall off your teapot.