This is a very common emergency and, of course, best prevented by keeping your dog on a lead and under control at all times when out of the home. Some dogs rush out the front door as soon as it is opened so get in the habit of putting your dog into a closed room before you answer the door.
If your dog is hit by a car try to stay calm and approach him very slowly and quietly, saying his name. Look to see if he is conscious and if there is any obvious bleeding or fractured limbs. His instinct may be to run and find shelter. If that happens follow him and let him settle, maybe under some bushes in a nearby garden. At least he is off the road. Sit with him and talk gently, reassuring him. Very slowly move your hand towards him and try to apply his lead but be careful handling him in case he may bite due to shock and pain. You may need to apply a muzzle - even a make shift one made by tying his mouth and then tying behind the ears with some bandage, a tape, string, a tie or a scarf. Make sure there are no breathing difficulties. Small dogs may be restrained with a towel over their heads while lifting them from behind.
Phone your vet as soon as possible and tell them you are on your way to the vet surgery. If there is a lot of bleeding, apply a pressure bandage to the wound using a bandage, clean tissue, cloth, or clothing tied tightly around the affected limb, or kept pressed down over a wound on the body. Lift by holding under the chest and the hindquarters. A small dog may be confined in a deep box or pet carrier. A large dog may be rolled onto a coat or blanket as a makeshift stretcher. Do not attempt to splint a fractured limb, just get your dog to the vet for the necessary attention. Use a blanket or a clothes item to cover your dog to maintain some body heat, as shock will make him hypothermic. Even if your dog is walking and seems okay he will still need to be examined by a vet, checked for any internal injuries and treated for shock and bruising.
You may also be shocked, so if somebody else can drive you to the vet surgery all the better. Get a hot cup of sweet tea or coffee into you as soon as you have handed over your pet to the vet’s care. Stay warm and rest until you are feeling better. In the sad case where your pet does not survive the trauma of the accident, or if the vet recommends euthanasia, try to contact other family members or friends to come and support you in deciding what you want to do. Don’t be rushed into making any big decisions on your own while you are shocked and feeling numb, or very distressed. Your pet can be kept sedated and free from pain until you are ready to decide what to do and have had a chance to alert other members of the family who would want to be involved.