I have not checked the veracity of the following, but I have been told that the French traditionally started their 300k brevets in the middle of the dark hours so that everyone could finish in daylight. Similarly with 400's, but perhaps they started those at dusk.
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I presume this is NOT a RUSA or ACP sanctioned brevet, because multiple starts are not allowed. I didn't know that until the NCBC 300k was considering options to deal with one of the horrendous southeast tornado storm 4 1/2 weeks ago. Even the RBA forgot. But a local randonneur chimed in on the local list-serve noting that they were pretty sure it wasn't allowed. Then, a certain Mark Thomas chimed in on the local (NC) list-serve noting that multiple starts were not allowed.

Actually, come to think of it, some starting on Saturday, others starting on Sunday was what was briefly considered; that was not within the rules. But I don't see how differing start times would be any different.