I entered Copperlane by travelling south from Caed Nua and using the Aedelwan Bridge. When I try to leave via the other gate the time estimate is clearly calculating the time to walk back out the Aedelwan Bridge, up to Caed Nua and round to the north end of the Madhmr Bridge, instead of the 15 minutes or so to reach the southern end. This is probably because I have not previously entered the Madhmr bridge zone from the north.

 

Ok it makes more sense now... perhaps that gate needs a helpful guard rather than a travel option?