English: An amazingly wide underpass/bridge in Auckland, New Zealand. This looks southwest under SH1 just west of the Newmarket Viaduct and Gillies Avenue. The costs to construct such a wide-span bridge strong enough to carry the 8-plus lanes of motorway above would be much more than for a narrower bridge/underpass fully suitable for the little traffic that uses this local road.

I can only imagine two reasons for this - either the rich & influential locals (the nearby hill area south of the motorway is very expensive territory) insisted on it, or there was a rule that when such a street was interrupted by the construction of the original motorway in the 1960s, then the road RESERVE (not the road) underneath had to be kept at the same width as before. This could also explain some grossly overwidth bridges over SH1 nearby, which are so excessively wide, they actually have parking ON the bridges.