Answer: He stole Loomis's hospital car.
Michael Myers had been hospitalized ever since he was six years old, when he stabbed his older sister Judith to death. On October 30, 1978, Loomis, his doctor, was supposed to take him before a judge, but Michael tricked Loomis and the nurse into getting out of the car. He then stole the car and went to Haddonfield, driving very well for someone who wasn't supposed to know how to drive!