Steve Hosgood wrote:
  

PS: Some US users might well have been entering weight limits in the 
"wrong" ton(ne)s already, though the percentage difference between long 
tons and tonnes is only slight. What sort of tons do they put on US 
bridge weight limits warning signs anyway?

  
    

It is unlikely that the numbers you see written for weight limits on 
bridges in different countries are in any way related as different 
building codes are used, and they define the weight limits differently. 
An imperfect world.