[USMA:51432] Standard unit for driving clearance

[Paul Rittman]

Occasionally I read of people complaining that they saw a sign that said that 
the clearance for a bridge was  2257mm or something like that. I realize that 
if you gave the figure in meters, you'd need to use the decimal for measuring 
the clearance of a bridge, or the lower level of a parking garage, etc. Is 
there a standard unit that is given for occasions like this?