Drivers often encounter sudden traffic backups on highways with no visible accidents or roadworks ahead. These frustrating halts, known as phantom jams, stem from simple scientific principles rather than external incidents.
How Phantom Jams Form
In dense traffic, vehicles travel closely together at high speeds. A minor event, such as a lane change or slight braking by one driver, triggers a chain reaction. The driver behind reacts by braking, prompting the next vehicle to do the same, and so on.
Human reaction times cause the braking intensity to amplify backward through the line of cars. By the 20th vehicle, the response escalates into a complete stop, creating a standstill with no apparent cause at the front.
The Backward Wave Phenomenon
Professor Hannah Fry describes this process: “When you are in that situation, you’re not stuck behind a crash. Your day has just been ruined by a mathematical ghost. The official name for this is a phantom jam, and we’ve known about it since about the 1950s, when mathematicians realised that when a motorway gets too crowded, people stop behaving like individual cars and instead, the entire motorway starts behaving according to the rules of fluid dynamics.”
Unlike traditional jams, phantom jams travel backward as a wave toward approaching drivers. The congestion dissipates only when the wave reaches a stretch of road with sufficient spacing between vehicles, allowing smooth flow to resume.
Preventing Phantom Jams
Maintaining a safe following distance eliminates tailgating, a key trigger for these jams. Tailgating qualifies as careless driving, potentially resulting in a £100 fine and penalty points on a license.
Professor Fry notes: “So how do you defeat this mathematical ghost? Two options, really. Either we can all get in driverless cars with much better reaction times, or we can all just agree to stop tailgating.”
Until autonomous vehicles become widespread, drivers caught in a phantom jam should remain patient. The wave will pass once it reaches less dense traffic ahead.
