What is causing my husband's fits of rage while driving?

Expectations. Your husband like, unfortunately many drivers, "forget" that they are sharing a road with others and have expctations that the traffic lights will always be green, or that other drivers are paying attention to his needs for expediency. His frustration can lead to "road rage" and speaks to a relative level of immaturity. Ask him to see a therapist before he becomes dangerous to you and others.