K.C.
My son is 2 and autistic.. He does this same thing, except it's when we back up. He freaks! There's not much you can do when you're driving, but once we start moving I reach behind and massage his legs while I'm driving with one hand and "shhsh" him and tell him "honey it's okay". Seems to work.
I don't know how to better the situation, really.
Just wanted to tell you you aren't alone.