Stop safely, stay out of traffic, and call 911 if the dog is on a busy road. Then treat it as injured even if it runs well, and get a vet to check it the same day.
Stop safely, then look
Pull over where it is safe, turn on your hazards, and note exactly where you are. Do not stop in a live lane or run across traffic. If you are on a highway or a fast road, call 911 or the highway patrol, say a dog is loose on the road, and give the road, direction and nearest exit or mile marker. Note the time. If your dog is in sight and near you, get low, call its name in a normal voice, and let it come to you. Do not run at it, because a frightened or hurt dog may bolt. Our post Dog on the highway: who to call and what not to do covers road calls in more detail.
If you can see your dog
Move only as fast as is safe. A dog that has just fallen from a moving car may be in shock, and even a gentle one may bite when hurt or frightened. The AKC says a muzzle can help when handling a hurt dog, but never put one on a dog that is vomiting. If traffic is close, the American Red Cross says to move a dog away from traffic before you examine it, if traffic cannot be stopped. A blanket or sheet can support a dog struggling to walk, and a flat, sturdy board can keep the neck and back still for a dog who cannot stand. Large dogs may need more than one person. Then drive straight to a vet.
A dog that gets up may still be hurt
Do not take running as a good sign. The Red Cross warns that internal injuries from trauma may not show for 12 to 72 hours. So plan a vet visit the same day even if your dog walks back to you wagging its tail. Call on the way so the clinic can prepare, as the AKC suggests. Tell them what happened: She jumped out of the rear window at about 30 miles an hour and landed on pavement. Give them your speed, the surface, and anything you noticed since then. Do not wait to see how it looks tomorrow.
If it ran away
Assume your dog is scared, and maybe hurt, and that it may not behave like itself. Practitioners at Missing Animal Response say that a dog in panic may travel far and avoid even people it knows, so do not assume it will come to its name. Go back to the exact spot and stay there if you can, or leave a person there. Ask a passenger to walk the side away from traffic while you call. Ask drivers and shops what they saw. Write down which way it went, and the time.
Who to call in the first hour
Work out who covers that spot, as it may not be your home area. Call animal control and the police non-emergency line for that town or county, then the shelters. Call vets and emergency clinics near the spot, since a stranger may take a hurt dog straight to the nearest one. Say this: My dog jumped out of my car at Route 9 and Elm around 4 p.m. He may be injured. He is a brown beagle mix with a red collar. Ask them to call your cell. Then call your microchip company, report your dog missing, and update your number if you need to. Add the location to every call. If the jump happened on an interstate, tell the state police as well.
Searching beside a road
Treat the road as the main danger, for you and your dog. Wear something bright, park well off the road, and do not walk on a highway shoulder or a ramp. Reach the land next to the road from side streets, parking lots and trails. Check ditches, culverts, brush and the backs of buildings. A hurt dog may lie low, so go slowly and look under things. Go back at dawn and dusk when the roads are quieter. Our post Dog missing after a car accident: searching the crash area has the roadside method in full, and all of it applies here. Put up large posters at gas stations and intersections close to the spot, and keep the words short.
When you get it back
Go to the vet, even days later, and even if your dog seems well. Tell the vet how long it was missing. Restrain it on the drive. The AKC says a crate or carrier is safest, and that a car harness or dog seatbelt in the back seat is the next choice. Use a leash and a secure collar or harness, because a frightened dog can bolt again from a parking lot. Then take down your posters and close your reports, so people stop looking. Lucky Dog Animal Rescue asks for this as well. Check your dog over gently at home, and call the vet if it stops eating, limps, or seems dull or sore.
Stopping it happening again
Dogs jump for things outside, and sometimes from fear, so change the car and not the dog. The ASPCA says to secure a crate so that it cannot slide or shift, and that if you do not use a crate, you should keep the pet in the back seat in a harness attached to a secured seatbelt. It also says not to let a pet ride with its head outside the window. Raise rear windows until the gap is too small for a head, and turn on the window lock so a paw cannot press the switch. Clip the leash to a seatbelt anchor before you open a door. Pickup beds need care too. California Vehicle Code 23117 restricts carrying animals in open cargo areas unless they are enclosed or secured, and other states have their own rules.
Sources
AKC, transporting a dog to the hospital: akc.org/expert-advice/health/how-to-transport-a-dog-to-hospital. American Red Cross: redcross.org/take-a-class/resources/learn-pet-first-aid/dog/car-accidents. Missing Animal Response: missinganimalresponse.com/lost-dog-behavior. Lucky Dog Animal Rescue: luckydoganimalrescue.org/articles/2024-08/ownership-resource/lost-pet-guidance. ASPCA, travel safety tips: aspca.org/pet-care/general-pet-care/travel-safety-tips. California Vehicle Code 23117: california.public.law/codes/vehicle_code_section_23117.