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Decision story

Why is the road ahead already moving?

Where should the speed be held when the jam has moving traffic at its front?

Rates lift payments. Payments eat buffers. Watch where that pressure concentrates.

Scroll · into the decision

With reduced motion, camera moves cut instead of gliding, the illustration shows the braked state without playing, and the beat narration is announced in a live region.

Hold the speed while the road ahead is still moving

This story ships as a directed film. Open /stories/where-should-the-speed-be-held/film.

One minute of lane 2, the brake that walks back through the cars, and the morning the window closes. The method, data and schema are at /stories/where-should-the-speed-be-held/method.

Methodology

Directed film over one frozen evidence pack. FHWA NGSIM trajectories for southbound US-101 on 15 June 2005 are read from the public SODA file. Cell speeds, the pocket's walk and the five-minute bands are arithmetic on those trajectories. A car-following replay of the featured minute is drawn only when the slow cell still walks upstream at about 9 mph and the far end stays in the forties. The opened cars are an illustration.

Limitations

One morning, about 2,100 feet, 15 June 2005. Positions are noisy. The entrance and the exit are drawn and not modelled. No variable speed limit was operating, so the film does not claim one would have worked. The film does not say why any particular driver braked.

Data references

  • [calculated] Lane 2, 240 s to 300 s: the slow cell walks 800 ft back, and the far end stays near 45 mph — 100-ft cells on observed trajectories. The featured car is 928, at 840 ft and 15.3 mph.
  • [calculated] The far end is near 40 mph for the first ten minutes, and near 23 by fifteen — Mean speed, lanes 1–5. Upstream is under 500 ft. Downstream is over 1,700 ft.
  • [calculated] The coarser rule counts a walk on lanes 2 and 3, in the first nine minutes — 200-ft by 20-second cells. A faster jump is not counted. Lanes 1, 4 and 5 did not meet the rule.
  • [modelled] A replay walks the slow cell back and leaves the far end fast; forbidding a harder brake does not — The replay slows the cell more than the road did, so its depth is not quoted as a measurement. One minute, one rule.
  • [illustrative] Three cars from the side: a lamp, a gap, and the next lamp later — The brake is enlarged. The speeds on the road are measured. Car 928's own acceleration at 240 s is positive.

Sources