When I was dumber and younger (but totally not now) I used to want for some kind of device that would let me display arbitrary real-time messages to my fellow drivers.

How stupid.

But you too. You've wanted to transmit to the other person in the lane next to you some urgent communique regarding their acute stupidity.

Well now! you! can!

What if your car window(s) were "frosted" just so. They just look a bit more opaque than normal windows. They're frrrosted!

Inside, on the roof of your vehicle, you have a projector (or something).

It projects onto these frosted windows.

You can correct for keystone and all that stuff and have a full-color high-resolution display onto which you can project your clever thoughts.