The Bell Law Firm, PLLC

The pictures of the wreckage from a series of accidents on Interstate 75 two weeks ago are horrifying. One survivor of the accident said it was like the end of the world, and the pictures bear that out. Debris is scattered across the road; the mangled remains of vehicles continue to smoke from a fire that was hot enough to burn a tractor trailer down to its skeleton.

Witnesses to the crashes said visibility was zero on that section of the highway. A nearby brush fire had sent smoke into the dip on the highway, and fog had rolled in. The conditions were bad enough earlier in the night for the state to close the road. But about half an hour before the accidents, the lieutenant governor had decided to reopen the road.

Florida, where this happened, has its own policies and procedures for determining if visibility is bad enough to close the roads. The area has experience with smoke and fog. The state patrol and state officials evaluate conditions, using information from troopers in the affected area and other weather data.

One tool is the National Weather Service's Low Visibility Occurrence Risk Index. The index uses a scale of 1 to 10 to rate the dispersion of humidity and smoke in the air. A score of 7 or higher usually means that the authorities should close the road. The Weather Service predicted an index of 6 the night of the accidents.

According to the National Weather Service, though, the index is not infallible. We'll explain further in our next post.

Source: CBSNews.com, "Few guidelines exist on when to shut down roads," Curt Anderson, Mark Carlson, Greg Bluestein and David Sharp, Feb. 1, 2012