Helicopters should not be allowed to travel over a four-mile stretch over the Potomac River following the deadly air collision near Washington in January, according to urgent safety

“We’ve determined that the existing separation distances between helicopter traffic operating on Route 4 and aircraft landing on Runway 33 are insufficient and pose an intolerable risk to aviation safety by increasing the chances of a mid-air collision at DCA,” NTSB Chair Jennifer Homendy said during a news conference Tuesday.