Ice caused hundreds of crashes across the area, and many freeways were shut down because they were too dangerous to navigate.
Drivers slid across the lanes and crashed into guardrails and other vehicles on almost every freeway. Houston police said more than 800 accidents were reported since 10 p.m. Thursday.
"We had an astounding number of minor accidents. We had more than 800 traffic accidents since last night with about half of those in the overnight hours," Houston Mayor Annise Parker said. "We've had a number of EMS and ambulance calls -- more than 700 calls."
Most freeways had sections closed before dawn because of icy conditions. Overpasses and bridges remained treacherous well after daylight.
Many drivers did not see the ice until they were sliding on it.
"It is horrendous. People really have to be careful while they're driving. If they don't have to go somewhere, stay at home," motorist Raymond Keith said.
Emergency crews had trouble getting to some of the wrecks because the roadways were so slick. Houston firefighters asked people to stay off the roads because they could not get to many areas.
"The ice that's there now really is not going to move, it's not going to melt away, until a little after the lunchtime hour. And it's not going to melt away completely because temperatures in the afternoon won't climb enough," KPRC Local 2 meteorologist Anthony Yanez said.
Yanez said the ice won't melt completely until late Saturday morning.
"By 9 p.m., we're going to head back below freezing again," Yanez said. "Tomorrow morning, we'll see that ice again."
"This is the worst of all possibilities," Harris County Judge Ed Emmett said. "People look out and say nothing happened, and they don't realize this is the black ice. This is the sheet of ice we were most worried about."
All Harris County toll roads were shut down Friday morning, including the Sam Houston Tollway, the Westpark Tollway and the Hardy Toll Road.
"We have the entire system, basically, shut down," said Peter Key, director of the Harris County Toll Road Authority. "It will remain that way until the latter part of the afternoon. I think there's a good chance that a majority of the elevated interchanges will remain shut down until tomorrow. If we're going to have another freeze tonight, and we can't get rid of the ice, we're just going to keep those closed until we get some warmer weather tomorrow."
Some drivers were stuck in traffic for hours because of all the wrecks.
Many drivers ran into trouble on the roadways and took extreme measures to try to get out of it.
"We're seeing people driving in the wrong direction on the highways or on one-way streets," Harris County Sheriff Adrian Garcia said. "We really need to urge the community to stay home. As Judge Emmett would say, hunker down. We need people to stay home or stay put. Don't get on the roadways. You're really going to put yourselves in harm's way; you're really going to put our brave Harris County sheriff's deputies in harm's way."
Garcia said that drivers could get tickets for going the wrong way.
"They likely could," Garcia said. "But our deputies are also using good judgment and recognizing that people are finding themselves not respecting the road conditions and weather conditions and they're having to make those dangerous maneuvers."
Many bridges on side streets were also covered with ice and should be avoided.
Houston police said there was an inch and a half of ice on the ramp from the South Loop to state Highway 288 North.
Houston's mayor stressed the importance of staying off the roads.
Parker said drivers should pay attention to law enforcement's instructions to keep everyone safe.
"Do not move the traffic cones," she said. "We've had a significant problem with that this morning. People want to go around those cones and then they get into trouble."