Why loose in the car is not an option
An unrestrained dog is at risk in three ways at once. In a sudden stop he becomes a moving weight that can injure him and everyone else; while driving he can climb into the footwell, block mirrors, or startle the driver; and at every stop he is one slipped door away from a car park or road. Securing him is therefore both a driving-safety job and a containment job, and it needs equipment that holds under force, not just a lead tied to a headrest, which can injure his neck and will not hold in a collision.
Choose the setup around your dog's size, temperament, and your car, and then use that same setup every journey so it becomes his normal travelling place.
Harness, crate, and guard options compared honestly
A travel harness with wide chest padding, clipped by a short connector to a seat belt anchor point, suits most dogs who ride on the back seat. It should fit snugly enough not to twist, let him sit and lie down, and never attach to his collar. A crate secured against sliding, sized for him to stand, turn, and lie flat, suits dogs who already rest calmly in one at home and gives nervous travellers an enclosed den. A rigid guard behind the rear seats keeps a boot traveller contained in his own space and away from the driver, but it does not restrain him within the boot, so pair it with a harness anchor if he rides there. Introduce your choice parked first and reward settling before any driving.
Fitting and positioning details that change the outcome
Keep him off the front seat entirely, because a deploying airbag is dangerous even for a restrained dog. Use the middle or side rear seat consistently, add a non-slip mat under him, and route the connector so it cannot wrap around a leg. For crate travellers, secure the crate to anchor points rather than wedging it with luggage, and never stack heavy bags where they could fall onto it.
Ventilation and temperature are part of securing him safely. Aim airflow at his area, use window shades against direct sun, and check him at every stop for panting or restlessness. A dog straining against a restraint, chewing it, or panting hard in it is telling you the fit, position, or his training in it needs work before a long journey.
Practise until the setup is boring
Build tolerance in stages: sit restrained in the parked car, then engine on, then a short loop, rewarding calm at each step. Teach a wait at the door so unloading happens on your cue, on lead, with the door opened only as far as needed. Practise at home and in quiet car parks before trying a busy service area, where doors, trolleys, and other dogs multiply the chances of a lunge or a slip.
Before every trip, do a thirty-second check: harness straps untwisted and snug, connector clipped to the anchor rather than a loose belt, crate latched and secured, guard firm, water and spare lead within reach for stops.
If the worst happens at a stop
Even careful owners can face a spooked dog bolting from a car door. Search the immediate area first, ask people nearby, and get word out quickly with a clear description while someone stays where he escaped, since many dogs circle back to the departure point. Contact local vets and wardens as you search.
For that moment, KinsLe Lost Pet Alert System sends a free alert to nearby members of the Circle of Love, and anyone who finds him can message you directly through his alert rather than relying on a poster.