Smarter Cambridge Transport

How do variable speed limits reduce congestion?

Which moves more people: a lane of traffic travelling at 70mph or 40mph? The faster one, right?

Wrong! It’s all down to safe spacing between vehicles.

Remember how the Highway Code sets out safe stopping distances at different speeds? These take into account the driver’s reaction time and the grip tyres have on the road. The Highway Code assumes reaction time is two thirds of a second, which is pretty optimistic. Even if the driver isn’t distracted, it typically takes longer than that to see brake lights on the vehicle in front, realise it’s necessary to make an emergency stop, and depress the brake pedal. A more realistic estimate for all that is one-and-a-half seconds (and it increases with age). In that time a vehicle has travelled 47m at 70mph, or 27m at 40mph.

Then there’s the physical braking distance. If your vehicle and the one in front both decelerate at the same rate, you don’t need much more spacing than required for your reaction time. However, what if the car in front runs into the back of a stationary vehicle? At 70mph, you’d need an additional 75m to avoid joining the pile-up. At 40mph, it’s 24m. (Incidentally, the chevrons you see on the M11 and other motorways are 40m apart. If you can see two chevrons, you’re at least 80m behind the vehicle in front – enough to avoid a collision in most, but not all, circumstances.)

Translating vehicle spacing into road capacity reveals that, if people maintain reasonably safe headways, a motorway can carry about 15% more vehicles per hour at 40mph than at 70mph. Of course, those journeys take longer, but the up-side is that more people can travel and still be safe. That’s why we have variable speed limits on many motorways.

Autonomous Emergency Braking (AEB) has an effective reaction time in milliseconds. If it can also ‘see’ the speed of traffic hundreds of metres ahead, it can prevent you running into a pile-up. In theory that makes tailgating safe – except how can you be sure whether the vehicle behind you has AEB or a driver with no road sense?


This article was first published in the Cambridge Independent on 13 January 2020.

Edward Leigh

Edward Leigh is the leader of Smarter Cambridge Transport, chair and independent co-opted member of the Cambridgeshire Police and Crime Panel, chair of the South Petersfield Residents Association, business owner, consultant, and occasional blogger about making the world and Cambridge a better place to live.

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