I've been to a local rally event for years now, marshals seem to be at every dangerous corner. Before the first pre-drivers come along (this year it was Walther Röhrl on a Porsche 953!!!) a car with a big red flag clears the track. If you try to cross the street after red flag you might get into trouble with a marshal near you.
I've never heard anything about whistles... would be stupid, hearing a whistle blow every minute, don't you think?
IMO it is stupid to cross the track during a race, those cars and drivers are always a bit above their limits and NOT trained to let people cross in front of them.
I feel sorry for the pilots of the car, those guys might be shocked for life, maybe never able to drive fast again.
greetz
der butz