Quick answer is : Do not think about it. Of course, this ain't possible all the time. Breathing is a good thing to do. Practising Mindfullness also helps. However, a true therapy with an certified therapist is the thing to do in my opinion. Be careful not to listen to advices that will end up aggravating the situation.
Another (cheaper) alternative is to avoid these roads, but this will obviously not help conquer your fear.