CHICAGO_A deadly accident in which a Boeing 737 slid off the end of a snowy runway brought renewed demands Friday for buffer zones or other safety measures at hundreds of airports around the nation to give pilots a wider margin for error.

In Thursday night's tragedy at Midway Airport, a Southwest Airlines jet making a landing plowed through a fence and into a street, killing a 6-year-old boy in a car. Ten other people, most of them on the ground, were injured.

The National Transportation Safety Board said the cause of the accident was still under investigation, and the plane's voice and data recorders were sent to Washington for analysis. But much of the attention focused on the 6,500-foot runway.

Like nearly 300 other U.S. commercial airports, Midway lacks 1,000-foot buffer zones at the ends of its runways. Midway, a compact one mile square, was built in 1923 during the propeller era and has shorter runways than most major airports, with no room to extend them because it is hemmed in by houses and businesses.

Safety experts say such airports can guard against accidents by instead using beds of crushable concrete that can slow an aircraft if it slides off the end of a runway.

The concrete beds - called Engineered Material Arresting Systems, or EMAS - are in place at the end of 18 runways at 14 airports. They have stopped three dangerous overruns three times since May 1999 at Kennedy Airport in New York.

"Certainly Midway airport officials should have already been trying to come up with something similar to this," said Jim Hall, NTSB chairman from 1993 to 2001. "There's really no margin for error at the end of that runway."