It used to be that the only term used was speed bump, but in recent years, speed hump also came into use. Someone decided there was a difference and signs needed to conform. I never bothered to look up the distinction, and still haven’t. But on the drive up School Hill on the Historic Bisbee Tour the other day gave me some perspective.
Apparently, if you’re going “up” the hill, it’s a speed bump, but once you’ve crested and are heading back down, it’s a speed hump. Makes about as much sense as anything else. Don’t know how that applies to flat streets, but there are few of those in Bisbee, so perhaps I don’t need to know.