The Art of Manliness recently wrote about how to jump from a moving car. After rolling down the window to reduce wind pressure and opening the door, jump away from the car, then tuck and roll:
Tuck. Before you hit the ground, tuck your body into a ball. Bring your chin to your chest and bring your arms and legs close to your body. This will prevent your brain canister from hitting the ground and spilling its contents on the pavement. Also, it prepares your body to roll.
Roll. Hit the ground with your shoulder and roll away from traffic. Rolling lessens the impact when hitting the ground.
For slightly different instructions, check out our past on article on stunts.