Pretty sure it was a memory problem from far too many cars being tracked. The road system was quite complicated with many sections of the city being long one way loops.
Long expressway journeys over water. Etc.
To fix the problem, as the city would not load again, I had to go to settings before loading and turn the two draw cars items off. I then removed the entire section of the onland expressway through the industrial section. I also removed the nuclear power plant (removes a lot of traffic) before turning draw cars on again. With the plant off, I proceeded to convert all one way roads to regular roads. One way roads aren't available when the town hall has no power.
I was having fun playing traffic cop. As the jam cleared in one section of the city, I would go to another section, and the traffic would be stopped there.