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← What is phantom traffic and why is it ruining your life?

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Showing Revision 4 created 05/22/2020 by lauren mcalpine .

  1. You’re cruising down the highway
    when all of a sudden
  2. endless rows of brake lights appear ahead.
  3. There’s no accident, no stoplight,
  4. no change in speed limit
    or narrowing of the road.
  5. So why the @#$%! is there
    so much traffic?
  6. When traffic comes to a near standstill
    for no apparent reason,

  7. it’s called a phantom traffic jam.
  8. A phantom traffic jam
    is an emergent phenomenon
  9. whose behavior takes on a life of its own,
    greater than the sum of its parts.
  10. But in spite of this,
    we can actually model these jams,
  11. even understand the principles
    that shape them—
  12. and we’re closer than you might think
  13. to preventing this kind of traffic
    in the future.
  14. For a phantom traffic jam to form,
    there must be a lot of cars on the road.

  15. That doesn’t mean
    there are necessarily too many cars
  16. to pass through a stretch
    of roadway smoothly,
  17. at least not if every driver maintains
    the same consistent speed and spacing
  18. from other drivers.
  19. In this dense, but flowing, traffic,
  20. it only takes a minor disturbance
    to set off the chain of events
  21. that causes a traffic jam.
  22. Say one driver brakes slightly.
  23. Each successive driver then brakes
    a little more strongly,
  24. creating a wave of brake lights
    that propagates backward
  25. through the cars on the road.
  26. These stop-and-go waves
    can travel along a highway for miles.
  27. With a low density of cars on the road,

  28. traffic flows smoothly
    because small disturbances,
  29. like individual cars changing lanes
    or slowing down at a curve,
  30. are absorbed by other
    drivers’ adjustments.
  31. But once the number of cars
    on the road exceeds a critical density,
  32. generally when cars are spaced
    less than 35 meters apart,
  33. the system’s behavior
    changes dramatically.
  34. It begins to display dynamic instability,
    meaning small disturbances are amplified.
  35. Dynamic instability isn’t unique
    to phantom traffic jams—
  36. it’s also responsible for raindrops,
    sand dunes, cloud patterns, and more.
  37. The instability is
    a positive feedback loop.

  38. Above the critical density,
  39. any additional vehicle reduces
    the number of cars per second
  40. passing through a given point on the road.
  41. This in turn means it takes longer
    for a local pileup
  42. to move out of a section of the road,
    increasing vehicle density even more,
  43. which eventually adds up
    to stop-and-go traffic.
  44. Drivers tend not to realize they need
    to break far in advance of a traffic jam,

  45. which means they end up having
    to brake harder to avoid a collision.
  46. This strengthens the wave of braking
    from vehicle to vehicle.
  47. What’s more, drivers tend to accelerate
    too rapidly out of a slowdown,
  48. meaning they try to drive faster
  49. than the average flow of traffic
    downstream of them.
  50. Then, they have to brake again,
    eventually producing another feedback loop
  51. that causes more stop-and-go traffic.
  52. In both cases, drivers make traffic worse

  53. simply because they don’t have a good
    sense of the conditions ahead of them.
  54. Self driving cars equipped with data
    on traffic conditions ahead
  55. from connected vehicles
    or roadway sensors
  56. might be able to counteract
    phantom traffic in real-time.
  57. These vehicles would maintain
    a uniform speed, safety permitting,
  58. that matches the average speed
    of the overall flow,
  59. preventing traffic waves from forming.
  60. In situations where there’s
    already a traffic wave,
  61. the automated vehicle
    would be able to anticipate it,
  62. braking sooner and more gradually
    than a human driver
  63. and reducing the strength of the wave.
  64. And it wouldn’t take that many
    self-driving cars—
  65. In a recent experiment, one autonomous
    vehicle for every 20 human drivers
  66. was enough to dampen
    and prevent traffic waves.
  67. Traffic jams are not only
    a daily annoyance–

  68. they’re a major cause of fatalities,
  69. wasted resources,
    and planet-threatening pollution.
  70. But new technology may help reduce
    these patterns,
  71. rendering our roads safer,
  72. our daily commutes more efficient,
    and our air cleaner.
  73. And the next time you’re stuck in traffic,
  74. it may help to remember that other drivers
    aren’t necessarily driving spitefully,
  75. but are simply unaware of road
    conditions ahead— and drive accordingly.