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She also explains how to avoid them.
A mathematician has explained why motorways like the M4 come to a standstill even when there is no crash(Image: John Myers)
A mathematician has explained why motorways like the M4 come to a standstill even when there is no crash. Hannah Fry explains: “One of the most baffling things about driving on the motorway is that sometimes you can be stuck in standstill traffic waiting to see the massive accident that’s ahead and when you finally get there, there’s nothing there, the road is just completely clear.
“When you’re 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… the entire motorway starts behaving according to the rules of fluid dynamics.
“Here is how the ghost is born: when the traffic gets really dense, the cars are driving quite close to each other, and maybe at some point somebody changes lanes or taps their brakes a bit too hard but, crucially, causes the person behind them to brake.
“That means the person behind them has to brake, the person behind them has to brake, and so on and so on. But because of human reaction times, the amount of braking increases as you go further back until you get to about 20 cars when the amplification is so big that the car has to come to a complete dead stop. And at that point you have set off this whole unstoppable force.
“The craziest thing about phantom jams is not that they’re happening in a specific location — you’re not driving towards a phantom jam — the traffic jam is a wave and it is coming back towards you. Once it forms, there’s nothing you can do, it’s just going to literally roll backwards through the traffic for miles and miles and miles like a giant, unstoppable shockwave.”
How does this shockwave end?
Hannah, who is an author, presenter and maths professor at Cambridge University explains: “It only died when it finally rolls on to a section of the road where the cars are spaced out enough that the wave an dissipate.”
So how do we avoid a “phantom jam”?
“Two options, really,” says Hannah, who was referring to motorways in general rather than the M4 specifically. “Either we can all get in driverless cars with much better reaction times or we can all just agree to stop tailgating. But in the absence of a minor miracle, there’s unfortunately nothing that you as a driver are going to be able to do. Sit back, take a deep breath and let the mathematical ghost wash past you.”
The ‘sub-standard’ M4
It may be that the M4 is particularly susceptible to traffic jams. Regular users of the motorway will be very used to slow and standstill traffic especially around Newport and Port Talbot.
In a report for the Welsh Government in 2018, Bryan Whittaker, a transport expert and fellow of the Chartered Institution of Highways and Networks, described the M4 as being of a “sub-standard nature”.
He said: “Some sections of the motorway have alignments that are below current motorway standards and in certain places lack a hard shoulder. There are frequent junctions, resulting in many weaving movements with vehicles accelerating, decelerating and changing lanes over relatively short distances.
“In the future… congestion will become more prevalent and traffic growth and the impact of incidents will have significant consequences. Journey times will become longer causing increasing frustration to the travelling public and have a negative impact on the Welsh economy.”
