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Wouldn't count on it, and if there are, expect long waiting times. Ernée is a very small town with around 5,500 inhabitants, so public transports are close to none. The population will increase exponentially that weekend. Obviously during the morning and especially after the races, everybody will want to leave town at the same time, so expect horrendous traffic jams. There's a public busline to Laval, but those hours are scheduled obviously and probably will be crowded as hell. As of yet there's no mention of extra transport for the event. I suppose you take the airplane to Paris? Easiest...