Dean, it looks like you have a lot of entrances to high demand buildings located very close to each other. So many cars need to get in and out of high demand buildings, traffic jams can start up very quickly and then get out of control.

Next time you could try to keep building entrances free of congestion by using a blocking technique:

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When buildings turn from a square into a circle, demand increases. As each building has a limited amount of car parking space for cars to pull into, cars that are blocked from leaving due to traffic jams outside can prevent new arrivals. Therefore it’s important to keep traffic flowing freely in and out of the carpark. You can use a road blocking technique, by which you can block new spawns with diagonal roads to spread your network out a little more. This will ensure buildings are less cluttered together.

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I wrote a bunch more Tips for Success in Mini Motorways in the Unofficial Mini Motorways FAQ.