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Moaning Kiwi They usually use that grey for a private road or otherwise problematic with mapping. Clearly that road isn't private though, however at the nearest junction it does warn of the height restriction at the next junction - where there appears to be a double low bridge. Maybe that caused a problem for the Google Maps camera vehicle at some point - though the section does appear to be fully photographed for StreetView.
Noisy Cephalopod that makes a lot of sense.