- untrained rider
- failure to use safety equipment
My conclusion is that a properly trained and equipped motorcyclist who stays 100% sober while riding is pretty safe. My 15 years of riding on the streets has borne that out -- I've had a few close calls due to insane drivers, but keeping up with safety training has given me the skills to deal. Without having taken the safety classes, no question I would have had multiple crashes.
The real issue is that even if you only get in as many accidents on your motorcycle as you would have in your car (which is probably nonzero, even if you're a great driver--other drivers suck, remember), each one is way more likely to kill you or cause serious injury when you're not surrounded by a ton of steel.
My stepfather (rode for 50 yrs) drove off a cliff and landed in a tree. My uncle was hit by a cyclist going through a red light and broke many bones in his hand (the cyclist's arm was severed!) My mother, a scooter driver, got her front tire trapped in streetcar tracks in the middle of traffic.