Source: http://www.mrporter.com/journal/the-knac...

You need to exit with your back facing the direction you’re travelling. Jump out with your right shoulder (remember, we’re on the left-hand side of the car) closest to the ground, and roll over your left shoulder. Once you hit the ground, the longer the time you can spend rolling over the biggest possible area of your body, the better: the key thing is to disperse the energy over time and surface area. Once you’re spinning and rolling, there’ll be a lot of centrifugal force so keep your body tense, prevent your limbs from flailing about, and go with the roll. Once safe, with just a few cuts and bruises (hopefully), you can dust yourself off and hail another cab.