[jubjubbird]

Seems like more often than not I see them folded up, not in use. Even with the price of diesel. I wonder how much they save?

[bluGill]

They are not allowed in all states/areas. So you might be someplace where they are not allowed, or close enough that there isn't a good place to stop and put them back out. I see them in use all the time, but that is a reflection of where I live.

[_ea1k]

Maybe 5% at best? Probably less, given that there's always some time spent at lower speeds too.