I'm sorry to be pedantic, but did you read what you copied?
First, MUTCD is an administrative manual that sets forth the relationship between federal, state and local government, not a public law that would give a private citizen a defense to a ticket. Second, I'd hazard to guess that in any particular jurisdiction, 90% of the road-miles are at statutory speed limits, so this section wouldn't apply. Third, and most glaringly, 85% of the average speed could never equal the 85th percentile speed. Unless the average you're talking about a mean that is being skewed to Hell by all the rocket-cars on the road today. (And can I get one?)
It just means that there's a consensus among civil engineers, that 3 speeding cars in every 20 an acceptable level of scofflawry.