Having biked there (once) it's simple. I apply the brakes on downhills. If you're riding on a bike trail with excellent visibility, you can go flat out. If you're riding on a 2 lane road with no bike lane, poor visibility, and no shoulder, you slow down.
When there are speeding vehicles and no shoulder or bike line, you are usually safer to speed up yourself. If the speed limit is 40, they'll be cars going around the curves at 50. Just after a curve is the most dangerous spot for getting hit from behind, and you really want to get out of there.
This might be true if the speed was not gravity-assisted and at the edge of the athlete's performance. Braddley wiggins can take it 35mph and ride with traffic on the flats. He can accelerate (up and down) on his own volition. 40 miles per hour is more speed than most can handle, in terms of decelleration, on 20-23c slicks high pressure tires with minimal surface area, to a terminal speed of zero. While modern brakes are good, they are not that good. Usually the solution is to swerve (avoidance), but in this case it appears there are no shoulders, and consequences off the Tarmac. It doesn't take 170+ IQ to figure out you need to respect your limits in the terrain--it has objective dangers. As an avid cyclist (and someone with experience in dangerous riding conditions) the arrogance of some others I see (both on the roads in in these comments) is surprising. It seems like there is some territorial issues at play. Regardless, one needs to respect objective hazard and their absolute skill levels as those risks rise.
