Please use common sense safety measures when traveling with your precious pooch on the road.  It is always best to secure your dog on a lap or harness when he/she is taking in the sights and smells.  Though most dogs will actually refrain from sticking  their heads out the window if your car is traveling too fast, don't even let them try on speedier routes like freeways. It may hurt their eyes, and increase the risk of injury with sudden slowing or stopping. 

I feel that a car ride is one of the few grand pleasures our dogs can experience in the human world they inhabit, so I, myself cannot bring myself to deny them this indulgence.  But again, be careful and use common sense!