I used to drive around with my dogs loose in the back of my Vanagon. I then watched a few doggie crash test videos. Horrified, I did a bunch of research.
I settled on the Kurgo Impact safety harnesses. As a pet owner who regularly drives with his dogs in the car, the videos are very rough to watch.
There are several other safety harnesses that I believe are as likely to help my dog in a crash. There are also a lot of safety harnesses that will not do ANYTHING for your dog and may make an incident far, far worse. I picked the Kurgo.
The Kurgo safety harness is crash-tested. In-person it is really solidly built and inspires confidence in an 'this is way more tough than the other one' way. The rubberized form-fitting pads are mated to the webbing in a way that it feels pretty bomb-proof.
Bomb-proof is what you need too. I do not recommend watching simulated dog car safety crash tests if you regularly drive with your pooch and aren't ready to own a hefty priced harness.
This video has a short clip of a crash test near the end and is not as gut wrenching as many others. The failure videos are awful.
I chose the Impact car-only harnesses that are tested up to 130lbs. It is easy enough to swap my pups from the Kurgo into the Easy Walk harnesses they normally wear for street walkin'.
For short rides, these harnesses are a pain in the ass, but they provide me enough peace of mind to drive the car at normal adult operating speeds on the few longer road trips I take.