The automatic emergency braking systems that will soon come standard on nearly all new cars don't reliably prevent crashes at the higher speeds at which the overwhelming majority of roadway deaths occur, a new study finds — and pedestrian-specific braking systems need further scrutiny, too.

Researchers from the American Automobile Association studied four major automakers' versions of the popular safety technology to the test, with a particular focus on the high-speed collision scenarios that are common on deadly U.S. roads, but uncommon in federal crash test labs.

At 40 miles per hour, things got dicey.