anecdotal I know, but a friend does triathlons and was wrecked bad enough that even with safety gear she was badly hurt. Even practices aren't safe and group riding always brings along risk. She was always surprised at riders who would join the group who somehow felt that riding a street bicycle didn't require safety equipment yet would not ride a BMX without the gear. Trees don't jump out at you any more than potholes, but they both can cause accidents and then you add in other riders and its not just you who has to be skilful.