I don't think I've ever used my left foot to brake in any car - manual or automatic - that I've owned. It just feels unnatural.

Anyway, I agree that anything taken to the extreme will result in reduced safety. Speeding is a method of trying to take time savings to an extreme, typically. The guy who says "Maybe I can get to work in 21 minutes instead of 25 minutes." Of course, your aspiring hypermiler could be driving distracted because they're juggling between closely watching the instant MPG gauge, tachometer for shift points, looking way down the road to see what's coming up, coasting as much as possible even possibly coasting with the engine manually shut off at high speeds, rolling through neighborhood stopsigns, tailgating in the name of drafting, and so on. All in the name of "Maybe I can get to work with 60mpg instead of 50mpg."

I think normal is the folks in the middle who aren't really speeding, but aren't paying any special attention to their technique, but rather pay that attention to a general awareness of what's around them, rather than what the car is doing fuel efficiency-wise.