Contact

The Momentum Papers · 05 of 15 · The shockwave effect

The shockwave cities can’t see

One vehicle slowing at one crossing can bring a whole network to a halt, long after the original cause has cleared. This is not a capacity problem. It is a physics problem, and wider roads cannot solve it.

The misdiagnosis

Congestion is not what you think it is

Most people believe congestion is caused by too many cars. Roads generally flow freely until traffic reaches very specific points. There, congestion forms suddenly and then travels backwards through the network. Those points are crossings and junctions.

Congestion is an interruption of momentum. Roads do not fail along their length; they fail at their conflict points.

Even wide, modern motorways can stop completely, not because they are full, but because their exits are stopped. The motorway has capacity. It lacks continuous flow.

~10,000Vehicles a day at a busy junction
~80,000At a very busy junction
100,000+At a stressed, failing junction

At a busy junction carrying 18,000 vehicles a day, stopping and restarting destroys about 14 GWh of energy a year, roughly what a 14 MW solar farm in Ireland generates. As volumes rise, the physics does not soften. It compounds.

How one stop spreads

Five steps from one car to a city-wide jam

  1. One vehicle slowsA signal, a pedestrian or a speed bump. The first vehicle brakes, and its momentum becomes heat.
  2. The next brakes harderReaction time means each following driver brakes a little earlier and harder.
  3. The wave runs backwardsEach vehicle adds a little more delay, and the shockwave travels back against the traffic.
  4. It spreads sidewaysLane changes, merges and shortcuts carry the disruption into neighbouring lanes and streets.
  5. It outlives the causeThe original stop may have cleared long ago, but the wave persists until the backlog is gone.
The slip road problem. When a signalised junction sits close to a motorway exit, queues back up onto the high-speed carriageway. Braking spreads across every lane, and late lane changes cause collisions far from the junction itself.

The mistake cities keep making

Managing interruption is not removing it

For decades congestion has been treated as a problem of scale, so the answer has been wider roads, smarter signals, digital routing and adaptive control. Congestion persists because these manage interruption rather than remove it.

What cities do

  • Widen roads to add capacity
  • Optimise signal timing
  • Add digital routing
  • Study traffic patterns
  • Manage the congestion

What cities need

  • Remove conflict points structurally
  • Keep momentum through geometry
  • Separate incompatible movements
  • Remove the need to stop
  • Remove the cause of congestion
Digital tools can observe congestion and redistribute it. They cannot remove its cause.