ANSWER: You have to fasten your lap belt.
Automatic seat belts were built into cars several decades ago (around 1970s) to try to solve the fact that not many people manually strap in the seat belts. However, the only thing that was automatic were the shoulder belts. These would automatically strap you in once you close the door or start the engine.
This means that you will still need to manually strap in the lap belts, since this component of the seat belt is still as important and could still get you in danger if you don't fasten it.