February 1, 2012

It took a few days to clean up the debris from the pileup on Interstate 75 last week. Cars and trucks had slammed into one another when fog and smoke combined to make it impossible to see your hand in front of your face. With 11 people dead and 18 injured, friends and families were anxious to know how this string ofaccidents could happen.

Visibility had been an issue all night. The state patrol had closed the road for a few hours; they reopened it about a half hour before the accidents. The first question for many was why they had reopened the road at all. The explanation raised another set of questions.

The National Weather Service produces something called the Low Visibility Occurrence Risk Index. The index rates the dispersion of humidity and smoke in the air on a 10-point scale. An index of 7 or more tells local authorities that the road should be closed.

State patrols use the index as a predictive tool. The night of the I-75 pileup, the Weather Service predicted a 6 score for the area surrounding the crash site — a close enough rating that state troopers were on alert.

It is the State Patrol that makes the decision to close a road, according to a representative in Florida, where the accidents occurred. Troopers in the area report visibility problems, and a supervisor at headquarters makes the decision. In some cases, if conditions are dangerous, troopers can close the road without approval. A representative from the patrol told the press that on-the-scene troopers carry a good deal of responsibility for these decisions — they are the “boots on the ground” who can see that, for example, they cannot see their hands in front of their faces.

The system has its critics. And we’ll go into that in our next post.

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


Categories: Blog