A professional stuntman offers step-by-step instructions. Some excerpts:
To get away with this in normal clothing, you’re probably looking at a maximum speed of 30 or 35mph.More at the link.
It’s important to escape when the driver is taking a bend.
Everyone’s first instinct is to put their hands or legs down first. That’s the worst thing you can do: you will break something... The best point of impact is the back of the shoulder and your back.
You need to exit with your back facing the direction you’re travelling.
Trying to land on your feet will likely result in your legs breaking, being whipped out from beneath you, causing you to hit your head.