Only for the record, there is a midway solution. Basically you go through a street making two walls on the two sides, using either piles or diaphragms, then yo

Only for the record, there is a midway solution. Basically you go through a street making two walls on the two sides, using either piles or diaphragms, then you excavate the whole width and build a concrete slab resting on the walls and restore the street surface, reopening the street/road to normal use. Then you excavate under the slab (that becomes the tunnel cover) and between the walls. This technique is fastish and safe, but as someone else commented in modern cities the amount of services (cables/pipes/etc.) you will find and need to modify, move, etc. may create really huge costs and use a lot of time.