Chasing is an instinctive behavior in dogs. they just love to chase and be chased. Dogs love going after cats, joggers, bicycles, cars, other canines and almost anything that moves. Moving objects stimulate dogs to chase. If your dog is a herding breed, then your his nature to herd the moving object is even greater.
Until you have learned to control your pooch's chasing instincts, do not let him off-leash. It's dangerous for him and the person or animal he is targeting. Before exposing your furkid to a situation where he will do that, you must train him in a controlled setting. It is important that you set up a situation where your dog can concentrate and understand the behaviour you want.
Start the training sessions indoors in your own home. Put him on leash and stand with him at one end of a room or hallway. Wave a tennis ball in front of him but don't allow him to touch it. Roll or toss it across the room or down the hall and tell him 'OFF'. If he starts to go after it, command "OFF!" and give him a firm tug on the leash.
It's extremely important that you do not allow him to touch the ball or he will think that the word 'OFF' means for him to chase and get the ball. Practice this several times a day until he gets the message that "OFF" means "Don't chase". When he gets it right, praise him profusely and give him a special treat.
When he learns this, repeat it in different rooms of your house. When it's clear that he really understands the meaning of 'OFF', try it without holding onto his leash.
Leave his leash on, dragging on the floor so you can grab it or step on it quickly should he forget what 'OFF' means. When he has mastered this, try it completely off-leash, but still in your own home and yard.
Try to practice with a jogger. Enlist the help of a friend to pose as one. Stand with your dog on leash and have your friend jog by repeatedly while you repeat the "OFF". Be sure your dog performs perfectly on leash before you try this off-leash.
If Fido is chasing cars, ask your friend to help you train you dog. Again, repeat the "OFF" exercise as your friend repeatedly drives by. Be sure your canine friend is perfect on-leash before you try it off-leash. It's important that you practice this in a controlled situation. You must know that the driver is aware of the training so he can stop the car should you lose control.
Practice with him daily and with every opportunity that arises. Praise him profusely every time you say the command "OFF" and he obeys. If he disobeys, give a strong tug on the leash, command "OFF!" and give him more practice. If Fido has a strong predisposition to chase, it is your responsibility to be alert for his safety and others. If you feel you cannot pay attention to the environment around you when you're with the him, simply do not let him off leash.