Runway design at US airports is under fresh scrutiny after an Amazon cargo jet tore past the end of a Miami International Airport runway on Sunday (video here), killing five and injuring five. As the New York Times reports, Miami complies with federal rules requiring a 1,000-foot safety area beyond where runways end—but unlike dozens of other airports, it does not use “crushable concrete” beds designed to stop runaway planes. Those engineered materials arresting systems, or EMAS, are in place at more than 70 airports and have safely stopped 26 overruns without deaths, according to the FAA. The agency has urged—but not mandated—their use for two decades, viewing them mainly as an alternative where space for full safety areas is limited.

Miami officials say that’s why they never installed an EMAS. “Runway 12/30 at Miami International Airport has the full 1,000-foot Runway Safety Area mandated by the FAA,” officials said in a statement. “EMAS is intended as an alternative, not an additional layer.” (WFLX notes that neighboring Boca Raton Airport, Donald J. Trump International, and Martin County Airport all use EMAS.) Now the National Transportation Safety Board is openly asking if Miami’s approach still makes sense. Chair Jennifer Homendy says whether EMAS should be standard “everywhere at every airport” will be central to the investigation into the incident. Some experts caution Sunday’s Boeing 767 was moving faster than EMAS are designed to handle; others argue it still might have slowed the jet enough to reduce the destruction beyond the fence line.