Ignoring largely empty stretches or highways where there's practically no signs, have you seen how many signs are out there on heavily occupied highways?
We're just coming out of Gulfport Mississippi, Highway 49 (don't worry I'm riding passenger, not driving), and it's pure information overload. When you're surrounded by like 500 signs, McDonald's, Circle K, RaceTrac, Shell, Chevron, this and that doctor's office, etc, and you're only looking for the BIG signs to see what exit to take, well its all too easy to miss the random speed limit sign that they don't even post but once every 3 miles or so.
When surrounded by signage information overload, while you're supposed to be keeping your eyes on the road, you simply cannot read every damn sign. You mostly keep your eyes on the road and follow whatever the pace of traffic is, only glancing up and around for the green signs with road names and highway/interstate directions.
The rule of thumb I've learned, is that no matter the posted speed limit, as long as you're driving safely about the pace of traffic, and aren't the idiot darting through 20mph faster than everyone else, then the cops generally aren't gonna mess with you.
School zones are definitely an exception, you better be going 15mph along with everyone else, and of course following the directions of the traffic guards and bus stop signals when to stop.
TL;DR - Too much information, cannot read all signs and keep eyes on the road simultaneously.
