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.
Traffic runs clockwise. At each of the 8 interchanges, an off-ramp leaves just before the cross street and an on-ramp joins just after it.
Each line is the last hour of that ring's path; the dot is now. The shaded hump is the best this freeway could do with perfectly spaced cars.
Left of the peak, the loop runs the other way: more cars means more deliveries, which drains the freeway back down. That's why the left balance point is stable. Right of the peak, every extra car lowers deliveries, so the freeway gains cars faster and faster.
The off-ramp itself is empty. The driver who wants it is stuck behind three drivers headed elsewhere. When the queue backs up past an off-ramp, that exit only works as fast as the queue moves, and on a ring every exit sits inside somebody's queue.
A car waiting on an on-ramp blocks nobody. A car waiting on the freeway blocks the exit for every driver behind it. Ramp metering moves the wait from the freeway, where it cuts deliveries, to the ramp, where it doesn't. The freeway stays near the top of the hump, and that peak is the most trips per hour it can ever finish.
The arithmetic behind the hump: