In a major US city? More a coping mechanism.
I was caught up in an evacuation situation on Hilton Head Island where a hurricane turned unexpectedly and the island was evacuated. We were literally packing up to leave for our scheduled departure, so we were close to the front.
Within 15 minutes, the roads were bonkers. Gas stations were out of gas within an hour, and the traffic was beyond insane took about 3-4 hours to get out.
This was in the Fall in a well connected vacation town, not even peak season. People were not panicking. The police and fire departments were present, prepared and professional.
If it were an initial war scenario, maybe 5% of people would get out, and once electricity was disrupted, the whole thing would immediately freeze.
The Swiss/Finn model is the only credible one and addresses only certain threats. They’re looking at protecting against fallout and conventional bombardment. All of the old US civil defense plans were designed around the notion of Russian bombers attacking US cities with atomic bombs, and said bombers getting intercepted by nuclear SAMs and nuclear air to air rockets. NYC, for example, was ringed with Nike batteries so in a war scenario you’d be looking at fallout (even if every bogey was intercepted) and and a disrupted power grid. It went to the wayside once the Soviets deployed ICBMs and hydrogen bombs.