Loose in the car is the default, and it is the worst option
An unrestrained dog distracts the driver, can be thrown forward in a sudden stop, and frequently escapes through an opened door at exactly the wrong moment on a roadside. It also complicates rescue: emergency responders arriving at a crash have to deal with a frightened, possibly injured animal before they can reach people. Every restraint system below is a large improvement over nothing, and choosing between them mostly comes down to your vehicle and your dog's size.
What crash testing actually certifies
There is no government standard for pet travel restraints in the United States, which means any product can print “safety harness” on the box. The meaningful signal is independent testing to a published protocol — most commonly a 30 mph sled test using a weighted dog dummy, run by an independent laboratory rather than the manufacturer. Look for the specific weight range the product was tested at, since a harness certified for a 25-pound dog tells you nothing about how it behaves at 75 pounds. Wide chest panels distribute force across the sternum; thin straps concentrate it. Metal hardware outperforms plastic buckles under load.
Option one: harness plus seat belt tether
The most practical choice for medium and large dogs riding in the back seat. Fit matters as much as the product: you should be able to slide two fingers under any strap, no more. Keep the tether short enough that the dog cannot reach the front seats or fall into the footwell, but long enough to sit and lie down. Clip into the seat belt latch or a cargo anchor, never into a collar — a collar attachment in a sudden stop concentrates the entire load on the neck. Note that a car harness is usually bulkier than a walking harness; many owners carry both.
Option two: a secured crate or carrier
A rigid crate strapped down in the cargo area is arguably the safest arrangement for a large dog in an SUV or wagon, and it doubles as your hotel and campground setup. The critical word is secured: an unstrapped crate is a heavy box that moves in a collision. Use ratchet straps to the cargo anchors, position the crate against the seat backs so it cannot slide forward, and avoid stacking luggage on top of it. Hard-sided carriers belted into a seat serve the same role for small dogs and cats.
Option three: barriers, and what they do not do
A mesh or bar barrier behind the rear seats keeps a dog out of the driver's space and stops it from being thrown into the cabin. That is genuinely valuable, and for many owners it is the easy everyday choice. But a barrier does not restrain the dog itself, which can still be thrown against the barrier or the tailgate. The strongest setup pairs a barrier with a tethered harness or a strapped crate rather than treating either as complete.
The rest of the back seat
A quilted hammock cover with side flaps stops a dog sliding into the footwell and keeps hair and mud off the upholstery. Add a non-slip pad underneath if your cover moves on leather. Airbags make the front seat unsafe for pets of any size, so keep everyone in back. A window shade prevents the greenhouse effect on long sunny drives, and a portable ramp saves the joints of older or long-backed dogs jumping down from a high tailgate — a small piece of gear that quietly prevents a lot of injuries.