General answer: Speed limits are set based on the 85th percentile speed of free-flowing cars. Page 3-16 of TxDOT's Procedures for Establishing Speed Zones manual says that speed limits on each side of a divided highway should conform to the 85th percentile traffic speed for that side; and speed limits on an undivided highway should be the same in both directions. In actual practice, they rarely put different speed limits on different sides of any road.

See below for commentary on your examples.

admin said:

I'll post more specific information soon; but, here's the general question I have:

Let's say you're going 40 mph on a road, northbound. Isn't the southbound traffic also supposed to be going 40 mph?

Yes, if the road is undivided.

Suppose you're going 70 mph coming into a town. Then the speed limit changes to 55 mph. At the point where the 55 mph sign is, there usually is a corresponding sign on the opposite side of the road that marks the increase in speed to 70 mph. In other words, aren't speed limits the same for certain sections of road, regardless of which side of the road you are on?! (with the exception of, perhaps, big divided highways and such).

Yes. And you correctly mentioned the possible exception of divided highways.

The specific area I'm talking about is on RR 620, from Mansfield Dam, south to the Lakeway city limit sign. They've made all kinds of changes there lately. And, now they've got a 65 mph limit on one side, and 55 mph limit on the other. Actually, the only thing causing the confusion is ONE 55 mph sign that never got changed to 65.

Was the speed limit recently raised from 55 to 65? And is the one 55-mph sign between two 65-mph signs in only one direction of travel? If so, they might have forgotten to change it. You can direct your concern to TxDOT, and if you are really curious, you can do an Open Records request to get the speed zone study data and strip maps from TxDOT. They conduct speed zone studies and develop strip maps to document speed zoning changes.

I'll post a detailed sketch very soon, but just wanted to get the question out there.