One measure is located in your car key. Modern keys are coded to match your car. When you put your key in the ignition switch, the car looks for that code. If you have the right code, your car will start. Without the correct code it won’t start. This defeats hot wiring, common keys and other methods that car thieves traditionally used to steal cars.
According to a NPR interview with NICB vice president Roger Morris, most modern day thefts occur because drivers leave their doors unlocked, with the keys in the car. Another common scenario is the driver parking their car at a convenience store and running in to buy something. Having left the keys in the car, they are just going to be there a short time; a thief easily takes the car away. A winter scenario in the Puget Sound region is a driver starting the car in the driveway on a cold morning at home to warm up while they get ready for work. A thief jumps in to drive the car away.
So the human is the weak link. Lock your doors and take your keys with you!