This is a classic case of what if thinking.
Could it be that you are anticipating the panic attack on the freeway way before you ever even set foot out the door?
Self talk I believe is truly the only way to calm yourself down and get through an otherwise unforseen panic attack. It is your negative thinking I believe that is still causing the issue here. If you were not thinking negatively you would not be panicing on the freeway.
I would also like to say that a distraction while you are driving is just that, a distraction and a temporary fix at best and relearning the skills in the program is in my opinion the only permanent way to get a handle on this.
How about going through the program and refocusing on getting really, really good at session #3 Self Talk the Key to a Healthy Self Esteem, and also session #8 Put an End to What if Thinking? Take these tapes with you when you travel and listen to them while driving on the freeway.
I still believe you are anticipating a panic attack and falling back into the what if thinking pattern here. You may very well not be doing this consiously (and who would do that to themselves purposely anyway?

It is possible that this is the only area left that needs extra attention and by revisiting the tapes you are doing just that. I would say good luck to you but luck really has nothing to do with it, you can do this and from your post you are almost home free

So happy trails to you!