Many motorists yield to pedestrians in crosswalks — but not when they’re driving at deadly speeds, according to a new study that shows the need to slow down car drivers with broader road design changes, and not just more signs and paint.

As part of a recent experiment conducted during more than 1,200 crossing attempts at intersections across four states, transportation professionals at Kittleson & Associates found that 75 percent of U.S. motorists will yield to a walker in a “basic” crosswalk, which is what engineers call intersections marked with nothing more than two parallel lines of white paint.

That was only true, though, when drivers were traveling at 20 miles per hour — a speed at which traffic rarely moves on auto-centric U.S. streets. Once motorists reached 30 miles per hour, just one in eight of them yielded to the walker in those same intersections.