The reasons of vomiting in puppy and in some human are same during a ride on car. It can be due to continuous looking outside when body is stagnant and surroundings are moving, this situation can cause some changes in body leading to nausea feeling and vomiting. Also the some animals are allergic to traffic smoke and cause vomiting.

 If you fed her within 2 hours of driving she will get car sick, put puppy always in back seat.