There are prescriptive uses and common uses less cars (prescriptively) is technically incorrect, however it’s common use and makes sense. Same with: “he can run 100 metres in fewer than 10 seconds” prescriptively correct, but sounds crap!
10 is uncountable, so less is correct.
Seconds are countable so fewer would be correct.
There’s fewer flour in the bag
There’s less flour in the bag.
When you try to set hard and fast rules like this, you’re pretty much bound to come up against instances where it just doesn’t work, hence my example of “fewer seconds” (prescriptively correct, not in common use)
Less cars on the road, or fewer cars on the road is just such an example, one will work just as well as the other.
Flour is uncountable so less is correct.
I can’t see the third example that you mention unless it’s the car thing. I imagine that the acceptance is because it sounds familiar, which is probably due to people accepting the wrongness of it for so long. Much like the addition of actual and literal into the modern language. Unnecessary and wrong (in most cases).