If you ever travel in a “heimish” bus you’ll see that the passengers are generally calm — despite traffic snarls. Besides for more options of stuff to do (espcially napping…) and being in public, sitting in the driver’s seat unfortunately gives one a false sense of control. When things get in the way and disrupt that notion — heavy traffic, other bad drivers etc., many simply lose it.
So basically driving is a good time to reinforce our belief that Hashem runs the world, we don’t.