Steve asked me if I knew the age of Bridge 166 over the Grand Union Canal. (It is the bridge south of Grove Mill, Watford. I need to do more work on the Canal pages and as yet don't have a picture (old or modern) of this bridge.
However a look at a map from the 1880s, and a comparison with a modern satellite view, shows the the bridge was built to switch the tow path from one side to the other. A cross over bridge has probably been at the spot since the canal was built over 200 years ago.