@John Brown - having a "feel" for your speed
Human beings are unable to properly estimate their speed without using clues like how fast the other cars are going. I'm sure you've noticed when you've been traveling at speed for a while when you reduce your speed you seem to be crawling. About 50-60 miles of my trip was on a state highway in Nebraska in an area with a population density of less than one person per square mile (yes, you read that right) I averaged about 110 mph on that stretch, at the end of which I had to slow down as it entered a town with a speed limit of 35. My brain was so fooled by the change in speed I felt I could have gotten out and walked faster than I was driving. While generally not quite so extreme, everyone has that disorienting sense of slowness when they've traveled at highway speeds for a while and then slow down a lot.
I'd defy anyone to be able to guess their speed within 10 mph if they sit in the passenger seat and travel a stretch of road with no other cars to provide frame of reference, or mile markers, etc. The reason you feel you can is because you do most of your highway travel in a fairly narrow range of speeds. You wouldn't be able to guess it so well if the driver slowly sped up to 100 mph, then dramatically slowed down, then quickly sped up to somewhere in between. You wouldn't know whether he was going 55 or 75 - you'd only be able to infer the speed from other clues.
You might know how fast YOU are going because you typically travel at around the same speed in reference to other traffic, because YOU know the area where you're driving. You have a pretty good idea what the average speed of traffic is in that area, and can guess how much faster/slower based on whether you are passing more often or being passed more often. Put you in an area where people typically drive 15-20 mph over the speed limit, if you don't know that, your guess will be off.