Random Texas Traffic Question

As long as I've been involved in cases that often begin with simple traffic violations, I've never had this come up: Are those yellow Texas signs (above) simply "advisory" or is it a violation to travel in excess of the designated miles per hour? I guess they could get your for reckless driving or some other offense, but the specific question is can they give you a ticket for speeding.

The DPS Texas Driving Handbook (pdf) - which is where I got the above graphic - doesn't really answer the question. (And there's no way I'm hitting the Texas Transportation Code when someone out there knows the answer.)

I brought this up to someone before, and they thought it was a no brainer that the signs were the equivalent of maximum speed signs on the highway. But if that's the case, why are they yellow?

Edit: And for you knuckleheads that think I don't understand the basic speeding statute, I at least educated myself four years ago.