Rights and wrongs of the motorway speed limit
The notorious recent road accident in Somerset has brought into sharpened focus the debate on motorway speed limits.
Despite the 34-vehicle M5 crash – the UK's worst since 1991, killing 7 people and injuring 51 – the speed limit may still be raised to 80mph.
However, asks Sue Vanden of Simpson Millar LLP, would raising the limit by 10mph lead to more road accidents?
"Logic says the faster you drive, the bigger the impact if you're brought to an abrupt halt," says Sue. "Although speed alone won't necessarily cause a road accident
, the severity of the crash
can be increased significantly if vehicles are travelling at high speeds."
In working out the potential results of an 80mph road accident, road safety experts need to assess many things. Besides speed, these include vehicles' weight, loads and conditions, plus the state of the weather and the road
Dr Lisa Jardine-Wright, a physicist at Cambridge University's Cavendish Laboratory, says a major factor is stopping distance
, which increases from 96m (315ft) to 120m (394ft) if the car's speed is raised by 10mph.
"For example, the stopping distance of a Ford Focus at 70mph is 22 car lengths and at 80mph nearly 28 car lengths. If a driver travelling at 80mph leaves a stopping distance designed for 70mph they will collide with the vehicle in front at nearly 40mph."
If a vehicle stopped immediately, the force felt from the seatbelt by a passenger or driver in that vehicle would be 33% more at 80mph than at 70mph. "Not a problem," says the director of the RAC Foundation Professor Stephen Glaister, "until you hit something".
"Suddenly all that energy has to go somewhere. Unfortunately, depending on the design of the car and the type of impact, much of it will be absorbed by the human body
Motorway accidents rarely involve single cars alone. Dr Jardine-Wright predicts that if a car travelling at 70mph without slowing down hit a line of closely-spaced stationary traffic, 3 cars in front would be affected.
"At 80mph it is likely that another car would be involved," she says. "The impact of more vehicles caught up in an incident could mean an increase in casualties."
According to Duncan Vernon, road safety manager for the Royal Society for the Prevention of Accidents (Rospa), speed does two things.
"Higher speeds result in higher-speed crashes and this can increase the severity of injuries. It also increases the risk of accidents. It exaggerates other errors, like following vehicles closely at higher speeds, which can lead to an accident."
He adds that drivers are uncomfortable driving at 80mph and agree that 70mph is suitable. Also against the proposal is the road safety charity Brake, which calls it "desperately inhumane".
All this said, the government still wants to press ahead. Announcing the proposal, former transport secretary Philip Hammond said the current limit, introduced in 1965, was outdated due to "huge advances in safety and motoring technology".
The government is supported by the Institute of Advanced Motorists (IAM) which says motorway users currently break the 70mph limit anyway.
However, Sue Vanden
concludes that consequences can go some way beyond a few single road accidents. "Other than the potential for far bigger road accidents as more cars become involved, it's important to bear in mind the likely strain on emergency services already being squeezed by budget cuts. Further calls on resources are the last thing that ambulance, police and fire services need at this time."