I think you'll find that all the coast roads will be closed - they'll open again once the last runners have gone past. So the promenade des anglais direction Monaco will probably close at 6:30 as the race starts at 08:00.
The roads used will be closed from 05:00 along the whole of the route and open again one hour after the runners have gone by and should all be open from 15:30 except for the Croisette which will be closed until 18:00.