I have found that my dog (who gets sick on especially windy roads) does better if she lays down. I have found on the way out of town on a trip (usually camping) she is very excited and wants to see all thats going on(and gets sick). On the return trip she is usually worn out from the trip and lays down and has no sickness what so ever.
I know that doesn't help but you may wanna ask your vet for a sedative for the trip