to tunnel or not to tunnel

building metro rail underground is done either by tunnelling or by cut-and-cover method. going underground has the advantage that it clears street-level for other use. that makes sense in high-density cities or high-density parts of cities.

but, what about cities or parts of city that are not high-density ones? it makes sense in these situations to abandon both tunnelling and cut-and-cover and go at-grade. many metro systems in north american cities do have this treatment, in parts of their system. at-grade LRTs are exactly that.

there is so much land space taken up by 6-lane arterial roads and they are all potential candidates to build at-grade mass transit. most often, the argument is that density doesn't support support high-order transit. but then, density can be created by converting space currently allocated to roads into high-density transport systems and then intensifying residential and commercial developments.

the cost of metro rail or light-rail systems reduce significantly when built at-grade, compared to under-ground or on viaducts. with driver-less and electric vehicles, perhaps the future of high-order transit is one that doesn't need overhead catenary or third rail for power source.