This has bothered me ever since I first saw it. As my husband (the nerdy engineer) puts it, it is just a matter of what will fail first/what is the weakest point. The tether, the buckle on the harness, the d-ring on the harness itself, the strap that holds the d-ring, etc. To my knowledge, the only successfully crash-tested car seat system out there is the Sleepy Pod, which is more like variation on a crate. Here are some of those videos:
Sleepypod - YouTube
I didn't see this link posted (may have missed it), but the Center for Pet Safety is the one running all these tests:
The Science of Pet Safety, Pet Product Testing, Crash Testing, Scientific Testing, Criteria, Pet Safety Advocate, PetSafety.
My girls are in Snoozers either attached with their car harnesses or Buddy Belts, but it sadly seems like that doesn't do much. I do agree it's better than nothing, and I really hope people who attach car seat tethers to the d-rings in clothing (not harnesses) will think twice about that too. I see that a lot, and it doesn't seem like that would hold up against much force at all.
