Icy roads are still causing major problems, leaving some North Texans to wonder why transportation officials aren't using salt to melt the ice.

Texas Department of Transportation and North Texas Tollway Authority crews treat roadways with a sodium chloride mix called MD-20 and a sand-and-10-percent-salt mix.

But MD-20 isn't as effective when temperatures dip below 8 degrees.

"They have to evaluate, according to maintenance crews, evaluate the situation as to whether they can use sand or the MD-20," NTTA spokeswoman Kim Jackson said.

TxDOT uses granular salt in some instances, such as when trucks get stuck on exit ramps.

"That rock salt will actually melt that ice very fast," TxDOT spokeswoman Michelle Releford said.

So why not use salt on all highways like some states do?

"It's highly corrosive, and that's why we don't use it in more of a concentration or in more places," Releford said. "We have more roadway than probably any other state, especially the East Coast, and we have to maintain it."

MD-20 is expensive, costing the state more than salt would, but TxDOT said it is cheaper than repairing the corrosion to roadways that salt would cause.

"We used salt more before the MD-20," Releford said. "The reason we switched to the MD-20 was because it was less corrosive."

The NTTA said it uses MD-20 for the same reason but may consider using salt during extreme weather events such as the one this week.