Researchers at the Saint Louis University School of Medicine found top-down driving at 55 miles an hour exposes driver AND passengers to 85 decibels of noise. That’s the equivalent of a gasoline-powered lawnmower. And the faster you go, the louder it gets. The experts say to put the top UP after driving at highway speeds for an hour. Long trip? Alternate an hour up with an hour down. Because prolonged exposure to anything over 85 decibels is known to cause hearing loss.