CHARLESTON, W.Va. -- State and local salt crews are doing their best to keep ahead of snowy and icy roads, but with temperatures expected to plummet below zero overnight, there's only so much they can do."When we're getting down to 20 and 15 degrees and below that, people are going to have to realize that it's a travel risk, and roads are going to be icy," said Carrie Bly, spokeswoman for the state Division of Highways.Charleston Public Works Director Gary Taylor said all of the city's 17 salt trucks will be out treating hills and intersections, but extremely low temperatures limit the effects of salt.As temperatures dip below 20 degrees, standard road salt quits working, Bly and Taylor said. Salt crews can add calcium chloride to the mixture to work a little better at lower temperatures, and add sand and cinders to provide more traction on frozen roads, but at some point, "It's just a losing battle," Bly said.The problem is that even if the salt mixture melts snow and ice, it's going to refreeze, making travel hazardous. "We're going to keep our trucks with plows on the road, and hopefully, if we stay after it, we can keep the ice broken up," Taylor said. "But sections of road that sit idle with no traffic will start to freeze."Our intent is to keep the trucks moving, especially on the hills."With almost 400 miles of streets in the city, Charleston street department workers don't treat streets in the flat parts of town, Taylor said. But intersections and hilly sections of town will be treated. In all, he said about half of the city's streets will be treated.Bly said state road crews do their best to keep ahead of snow and ice but can only do so much. She urged motorists to be careful driving when temperatures are so low, or not go out at all.Taylor agrees."We encourage people to use caution," he said. "If they don't need to be on the road, they shouldn't be on the road."Going to visit someone or going to get a loaf of bread aren't reasons to be on the road," he said. "The best thing I can tell people is to stay off the roads unless it's absolutely necessary."Staff writer Lydia Nuzum contributed to this report. Reach Rusty Marks at email@example.com or 304-348-1215.