Da Groj

It is entirely possible to whack your helmet on your rolled up window. While the window may likely laugh that off, no sense risking a shattered window. Plus, we can't yell at you if we need to get your attention if windows are up. Then, we are going to have to walk all the way over to your car and wave our arms at you. Frankly, that sounds like effort.
You can get helmets with remove-able padding for easy cleaning.