Vomit happens. Sometimes even humans get carsick. Most dogs can overcome motion sickness through desensitizing them. Gradually accustom the dog to spending time in the car with the engine off, then with the engine on, then short trips, then the cross-country adventure. Prior to a trip be sure the dog has been fed at least three hours before you set off. You can also use anti-motion sickness medications to help settle the stomach and prevent the sometimes prolific drooling that occurs in a nauseous dog. Most medications are very safe antihistamines and many dogs eventually can travel without the aid of medical assistance. Just in case, bring a roll of paper towels.