I've found a story in the Chron archives which definitively answers the question about the existing cameras at certain intersections inside the Loop. Basically, Kevin was right - they're there to facilitate traffic flow along certain roads, though they're not specifically tied to the US59 roadwork. Here's an excerpt:

The city began using the Video Image Vehicle Detection System more than a year ago in the Reliant Park and Texas Medical Center areas. Since then, the number of cameras in the city has expanded, and Montrose, Westheimer and Richmond are three main roads where they are prominently featured.

"We have a few dozen of them around now and we'll be installing more throughout the coming months," said city of Houston Public Works spokesman Wes Johnson. "We've resynchronized all of the major corridors in Houston and as we get more money to install more of the cameras , we'll do so."

The city's Traffic Signal and Timing Optimization Program began in January as part of Mayor Bill White's plan to increase mobility throughout the city.

According to Public Works' Web site, the TSTOP program will affect more than 1,500 traffic signals in the city on 85 corridors, in areas including downtown, Midtown, Uptown and the Medical Center.

[...]

The cameras are used to count cars that go through light cycles at intersections during off-peak hours, so the city can potentially make programming changes at those intersections if they're needed.

The cameras can also prompt the traffic signals to change based on who is waiting at each light.

Four cameras are in place at each of the intersections, one monitoring traffic flow coming in each direction.

What the cameras do not do, Johnson said, is record what vehicles do at the intersections. The way they're set up now, he said, is such that even if the police or the city wanted to use them to monitor drivers' behavior, they couldn't.

"We haven't even gotten close to that," he said. "They've been set up intentionally so they don't pose any invasion of privacy. They can't see drivers, and they can't read license plates. When you get into looking at license plates or drivers, that's a gray area."