R.C.
I like Maura's suggestion and would add: If you're pulled over in a place where it's safe to do so, and she continues to scream, you should feel free to step outside the car and close the door. Tell her she's hurting your ears, and when she's done screaming you'll be able to continue driving. The little buffer against the noise will make it easier for you to wait her out. She really needs to know this is not negotiable - for the safety of your whole family.
Good Luck!