Having worked on these sorts of events myself, I have some experience. The rolling road closure starts about 15 mins ahead of the race with police motorcyclists stopping oncoming traffic and pulling them over to the side of the road. Side roads are closed off initially by police then by NEG motorcycle marshals. As the race moves forwards, the police stay ahead of the race. Once the riders, official cars, team cars and the rest of the entourage pass you, there is a police car at the back, after which the road is open.