Although very unsafe behavior, chasing vehicles is very worthwhile for the dog. Man's best friends loves to go after anything that moves, and the faster it runs, the better.  To crack down on this practice, you must negate the benefits of the hunt. Chasing vehicles is mostly done by canines that have free reign of the whole property and don't see many vehicles.

Expose your doggy to traffic by ensuring that you walk on busy streets periodically.  Command him to be still in a sit-stay position as vehicles pass, and move the collar back if he tries to move.  You can set your four legged friend up for a car chase retraining session by having a friend ready with various cans filled with pebbles, for a noisy effect.

Have your friend drive up to or past the place where your dog begins to chase cars. Tell your friend to apply the brakes as fast and as noisily as possible, when your dog is at a full run after the car. When the car does come to a noisy stop, get your fellow dog trainer to jump out of the car and create a stupendous din, by throwing the cans near your four legged friend. Dogs don't like loud noises generally.

If your dog is not likely to snap, have your friend aggressively run after him, herding him home, and throwing tins by his rear. Make sure you don't hit your dog however.

Repeat this setup with different automobiles, and if possible, other friends, until the recalcitrant canine stops chasing cars.  You can also use this retraining session when man's best friend acquires the potentially dangerous habit of chasing bikers, joggers, and horse riders.