I don't know if the helmet had much or any positive effect, but anybody who has followed and participated in cycle racing for years knows that crashes at 40 mph are extremely common. This is a slow speed for the finishing sprint or any modestly downhill section. Racers would usually jump back on their bikes and finish the race after a crash at that speed. I've seen this hundreds of times in local as well as pro races. Until 2003 European pros didn't wear helmets. The only fatality I heard of since the late 80s when I starting following the sport, was Fabio Casartelli who slammed head first into concrete pole at 50+mph. In that situation a helmet would not have helped.