One brief tap on the brakes can bring more than 20 vehicles to a halt on a busy motorway. This can happen without an accident or roadworks in sight.
A simulation of the M25 at junction 15 during a Monday morning peak shows how close following distances create phantom traffic jams. When drivers sit roughly one second behind the vehicle ahead, a single brake tap by one driver causes a ripple effect. Within a minute, traffic grinds to a standstill for dozens of cars.
How phantom jams start
Cars travelling close together allow drivers less time to react. A light brake tap by one driver is detected late by the following driver, who brakes harder to compensate. This intensifies with each driver behind, leading some to stop completely. The wave of braking travels backward through the traffic, even as the original driver resumes speed.
This effect was demonstrated in 2008 by a Japanese research group led by physicist Yuki Sugiyama from Nagoya University. Twenty-two cars drove at a steady 30km/h on a 230-metre circular track with no junctions or bottlenecks. Still, a small disturbance spread, causing brief standstills. Sugiyama noted that small jams on the track behaved like those on real roads.
Two seconds makes the difference
The M25 simulation ran again, this time with drivers maintaining a two-second gap. The same brake tap was applied. Drivers had enough space to ease off the accelerator instead of braking sharply. The ripple of braking faded out quickly and no vehicles stopped.
The two-second gap does not increase travel speed. Instead, it prevents jams forming in the first place.
Legal and safety context
Rule 126 of the Highway Code advises drivers to keep at least a two-second gap behind the vehicle in front on high-speed roads and in tunnels where visibility is limited. It recommends doubling that distance on wet roads and increasing up to ten times on icy surfaces.
The two-second gap can be checked by picking a fixed roadside point. When the vehicle ahead passes that point, start counting. If you pass it before counting two seconds, you are following too closely.
Following too closely is a significant safety issue. In 2018 National Highways found that one in eight road casualties results from tailgating, causing over 100 deaths or serious injuries annually. Their survey also revealed that tailgating is the most common complaint drivers have about others.
National Highways launched a campaign to reduce tailgating last year, highlighting its risks.
About the simulation
The M25 simulation uses a standard driver behaviour model. It illustrates how following distances affect traffic flow under typical peak conditions. It is not a recording of actual traffic but provides insight into common causes of phantom jams on UK motorways.
