What? Where do they do that? If they did, then that's pretty crazy. How can they say the "bridge can support 5 tons", but it would only support 1? That would destroy the bridge when some large vehicle tried to pass over it.
Actually, the system they use measures only the useful load supported by the bridge, not the road surface and other bridge components. It's really quite elegant.
Here, Calvin's dad can explain it better than I can:
