I've hit 40 on my mtb and 43 on my road bike. The mtb felt much safer, but it was more fun on the road bike. I suppose you've just got to accept that you could get messed up pretty bad if you were to go down... That said, I'm concerned more with cars than crashing @ speed. As long as your bike is in tune and you can handle your steed, I wouldn't worry. Just watch out for obstacles.