Some years ago, some states or municipalities or something tried that, or I remember seeing signs with metric figures, but the idiots that made the signs divided down to three significant figures - like, "So-and-so exit:
1 1/2 mi. (2.41 KM)" or "Speed limit 45 MPH (72.4 KPH)" and they just made people laugh, so they abandoned it.It's also surprisingly expensive to put in a new road sign.
Cheers! Rich