Everyone could just get into one of the 2 cars they own and cool down intermittently. Those that can’t could ride a bus or ask a friend. Some people would choose poorly but most would mitigate the worst heat.
While that might be true of Phoenix (if the cars aren't electric, and gas station pumps continue to work during this hypothetical blackout), US car ownership rates are _insanely_ high compared to the rest of the world. India is a good example, where car ownership rates are about 12%.