A freeway's output depends on how many cars are on it. Past a tipping point, adding cars means fewer of them reach their exits, and a ring freeway can lock solid. Both rings below get exactly the same drivers. The left one lets everyone on; the right one meters its on-ramps. Raise the demand and watch where the cars end up.
Runs entirely in your browser, so nothing is sent anywhere and there's nothing to install. On a phone, the full screen link above is a bit easier to work with.
Please feel free to use this in your own work. If you'd like to talk it through, or think a version built around your own data would be helpful, I'd love to hear from you.
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