Back last summer coming back from an evening 'shop group' ride, we all made a subconscience decision to up the pace a bit, as we hurtled along, someone in the group glanced down at the computer and in a matter of fact way informed the rest of us that we were going along at 22mph, we were tightly packed as well due to the character of the road, at that sort of speed, you start to wonder what mess you'd end up in (how much skin) you would lose if there was a crash...scary!
When you get into that sort of situation though it's not obvious that you are putting that much effort in, as in, if you were out solo and doing that speed, you would feel the effort much more.