Use Safety Measures
Consider a Hidden fence to help reinforce the idea that if the dog is un-monitored it will still be safe from the road.
Monitor Outside Play
Always try to keep and eye out the window and check here and there make sure your dog is safe, and away from harm.
Attention and Exercise
dogs are inclined to chase cars due to the pent-up energy, combined with the natural desire to chase. Giving your dog extra attention and plenty of exercise, can help limit this extra need to chase.
The above steps can be done immediately, to try to correct this behavior moving forward, it is important that your dog masters the “Come” Command and learns to obey his owner.