7: Residential Speed Reduction

“Twenty‘s plenty for us.”

When a car hits a bicycle, regardless of fault, the bicycle and its rider will come off worse. Cars have been made progressively safer over the years with seat belts, air bags and anti-lock braking, yet at the same time their weight has increased. It is the weight and speed difference that causes the injury to cyclists rather than car drivers.

Given that the kinetic energy in a car is transferred to a bicycle when they collide, reducing the kinetic energy of that car before it hits the bicycle is key to reducing the severity of car and bicycle crashes. The kinetic energy of a car is proportional to the square of its speed. For example, a car travelling at 40mph has four times as much kinetic energy as a car travelling at 20mph, even though it is travelling only twice as fast.

A simple way to reduce the severity of collisions between cars and bicycles is therefore to reduce the speed of cars. To do this, blanket 20mph speed limits should be introduced in areas where there is insufficient space to provide a high quality cycle lane. These speed limits are most appropriate in residential areas, where children may also be playing in the street.

Within Cambridge, the residential areas of Coleridge and Newnham would be excellent examples of where these safety measures can be introduced.

With any reduction in speed limits, the ability for a car driver to recognise that a given location has a 20mph limit is important. It is therefore proposed that at any junction between a 20mph and a 30mph road the pedestrian footway and any cycleway should be continuous across the junction. This would mean that the car leaving the 30mph limit would have to travel over the pedestrian footpath giving an unambiguous indication of a change in priority.

This would also mean that pedestrians would have explicit priority over cars turning into the side streets, increase the safety for pedestrians, and making walking a viable alternative for many people.

Implementing 20mph zones in residential areas throughout the city will make little difference to a driver’s overall journey. This is because only the edges of the journey would be affected – the rest of it would remain as 30mph sections. Yet, walking and cycling would become hugely more attractive.