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Boston has been cursed with some of the nation’s worst traffic. These days it is often a basic math problem — too many cars and not enough road.

But traffic jams are about more than volume. Drivers make choices that have lasting ripple effects. A glance at a smartphone — changing the radio station. In that distracted moment, one car brakes. Then another. Brake lights cascade backward.

There are plenty of pinch points. But the Southeast Expressway seems like another level of hell.

In the video above, we take a look at one specific place where backups often happen, and how, on bad days, the traffic spreads like a virus.

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Read Part 1 of the Boston Globe Spotlight report, “Seeing Red.”