Getting that many people out of the death zone is not a matter of a "short time."
We may need to rethink a LOT of policies--such as the desire to stuff as many people as humanly possible into close quarters and call the result "urban planning."
A ground burst would not cause nearly the fire damage that an air burst would because the most of the city would be shadowed by the nearest buildings.
But the fires that DO start will not get put out, and will continue dumping secondary activation products downwind, complicating evacuation efforts (and, incidentally, dumping those products in now windowless buildings that you're using as fallout shelters.
The evacuation process itself would kick up an immense amount of radioactive crap for everyone to breathe in.
I agree. With the technology we have, cities are obsolete. Dispersal of the population is a good thing that is being disparaged as "suburban sprawl" by urban politicians who see it as a shrinking tax base.
But the fires that DO start will not get put out, and will continue dumping secondary activation products downwind,
There has never been a nuclear ground burst in city before so we can only speculate. Anything shielded by a building would not catch fire and would receive a lot less blast damage. It would not be a firestorm.