When a train carrying tank cars of ethanol derailed Friday and caught fire in Winnebago County, Ill., officials decided to use the county's emergency notification system to warn residents rather than newly installed sirens. The reason, according to an article in the Rockford Register-Star, was to avoid confusion and to limit the warning to a smaller area than the sirens' three-mile radius.

If residents thought the siren was for a tornado, for example, they might hole up in their basements, rather than vacating the area. The sirens have three warning messages: A continuous tone means severe weather or tornado. A six-second rise and fall is the signal for a terrorist attack, and the evacuation tone rises for 16 seconds and falls for 8 seconds.

Wayne Hanson  |  Senior Executive Editor, Center For Digital Government