I wasn't going all that fast to begin with ... maybe 15-20 km/h as I rounded the last corner because I was doing a lot of skidding. I did pick up a bit of speed as I started down that last straight stretch, but then slowed a bit when I noticed that the intersection was a 4-way stop. I was probably back down to about 15 km/h when I crashed.
then it sounds like the combination of grade, rain and slick mud was what probably got you to lose control. 15-20 km/h doesn't sound like the sort of speed where one would lose control, especially on a straightaway, but add some mud and non-knobby tires and all bets are off.
I was having nightmares on the very first night, shortly after we set off!! It POURED from shortly after the water stop all the way to Mortagne, and I was crawling down most of those hills at about 8 km/h.
I left St Quentin around 10:10 and the rain was constant but not pouring. I do remember almost losing control somewhere in the first 10km after hitting a small berm that was obscured by all of the cyclists ahead of me; but otherwise the ride to Mortagne was wet but not too terrifying. However, on Wednesday night, the rain resumed, and I remembered riding my brakes on one long descent into Loudeac, then being passed by a tandem easily doing 50 km/h (I was doing ~30+ and they passed me like I was standing still) and all I could think was that they either knew this course like the back of their hand or the sleep deprivation was making that team utterly suicidal.