First and foremost, you must consider the safety of yourself and the other motorists. Never stop unless it is safe to do so. In our experience, the best thing you can do is to try to get a picture (even if it is far away) and a good description of the dog (size, weight, color, hair length) and report it to the local authorities. Note the exact location and time of the sighting.
Any attempt to pursue or capture the dog puts him/her in grave danger and may end up in tragedy. If an owner can be located they can take the necessary steps to capture their dog safely – away from the busy roadway.