Good evening - I'm Dr. Sara. I'm a licensed veterinarian who works exclusively with cats and dogs. I'm sorry to hear that you've had such a rough day today! I know that it's so scary when you see them get hit by a car!
The road rash and any small cuts or scrapes are usually less of an issue and in a young dog typically heal just fine. What I see most commonly in my patients hit by cars is pain and shock, manifested by high heart rates, pale gums, and overall depression or weakness. The treatment for that is IV fluids and good injectable opioid pain medications. Oftentimes just one dose of good pain relievers can bring them around and make them feel so much better. The relaxation caused by these drugs is very helpful too, since they are just as scared or even more scared than you are!
One big concern after a hard blunt trauma like that is for lung contusions, which are basically bruises on the lungs. In some cases these bruised areas can interfere with effective breathing and even be life threatening. They also don't all develop immediately, but rather develop slowly over the first 24-48 hours after the initial incident. I always take chest X-rays on my patients who were hit by cars for this reason. We also monitor their respiratory rate, character, and oxygenation levels quite closely.
Any hit by car incident really merits a trip to the vet, if nothing else just to make sure that there's no internal bleeding or lung contusions and to begin treating with pain medications. With wounds, I usually also prescribe antibiotics to prevent secondary infections. Watch her especially closely for labored breathing, pale gums, or a respiratory rate of more than 40 breaths per minute. These could indicate serious internal disease that can be life threatening.
Please let me know if I can answer any other questions.