Many measurements we use aren't ideal - such as feet, inches, miles, gallons etc. but many of us are more comfortable with those and understand them and what they mean, so we stick with them.
We aren't going to change all our road signs to kilometres just because km is a 'better' measurement than a mile. (I hope)
RE: Roadsigns.. we probably will soon. We said we'd never switch from imperial weighted measurements in the 90s..