I believe that given enough money and effort, that most modern cities CAN be evacuated.
It would take some days and be hugely expensive, but in most cases I believe it can be done.
This of course presumes that some relatively slow moving disaster such as a hurricane is approaching.
In the case of a disaster that gives little warning, such as a tsunami, I doubt that evacuation could be completed in time.

In the case of a nuclear detonation without warning, it is more of a case of rescuing the survivors rather than evacuation.

Martial law might be needed to keep traffic moving, requisition vehicles and fuel supplies and so on.
And of course most of those evacuees will need shelter and supplies, a non trivial undertaking.
Shopping list for sheltering evacuees.
2 million folding or inflatable beds
6 million blankets.
10 million bottles of water.
10 million MREs.
250,000 tents each to sleep 10 persons
250,000 portable lighting units
25,000 generators each 20KVA.
250,000 chemical toilets.
2 million rolls toilet paper.
And many other supplies, fuel, medicines, showers, soap, towels, clothing, soaps and detergents, tableware, communications equipment, Heating equipment and fuel, and so on.

And enough secure storage space to keep it all, outside the disaster zone, but not too far away.
Enough vehicles to transport it all, a few thousand trucks should do.

Remember also that trying to move a few million people will lead to significant loss of life from auto accidents and other mishaps.

Planning to shelter in place might be more sensible.