Why do they post the height of an overpass on a big sign on the side?

[Frank Cannon]

When, by the time a truck that is too tall gets to the over(under)pass, it's way too late to do anything about it?

I figure someone here will know.

[Kutjara]

1. Probably so they can fine the trucking companies large sums...

...of money for blocking the road when their drivers can't get under the bridge and can't turn around. Scams like that are nice little earners for local government.