Self-driving cars could more than double road capacity

The reduced safe headway – the gap between two vehicles on the road – needed for self-driving cars could more than double the capacity of existing road networks as well as significantly cutting carbon dioxide emissions.

Google is developing a self-driving car which it hopes to make available to the public in 2020.
Google is developing a self-driving car which it hopes to make available to the public in 2020.

In the latest issue of the ICE Engineering Sustainability journal, Ben Hardy of Cognizant Technology Solutions in London and Richard Fenner of Cambridge University say cars fitted with 'cooperative adaptive cruise control' could safely drive significantly closer together than those with human drivers.

No more 'stop and start'

The UK Highways Agency recommends a two-second interval between vehicles at the national speed limit of 70 mph (113 km/h), which equates to a 62m spacing. Allowing for real-world vehicle-to-vehicle communication lags and braking differences, the authors calculate that a 24m gap is safely achievable.

They then applied the headway to a highly congested and polluted stretch of the A14 between Huntingdon and Cambridge, and found it could safely increase traffic flow from the design capacity of 3,600 vehicles per hour to 9,213 vehicles per hour, an increase of over 250%.

During peak flows on the A14, traffic currently operates in 'stop and start' mode and an average speed of just 20 mph (32 km/h). The authors say autonomous vehicles would also be able to maintain steady flow conditions at this speed, cutting greenhouse gas emissions by 43%.

Government action needed

However, to make it happen, government action is needed. 'Transport policy plays a vital part in driving the tasks necessary to enable high-volume deployment of cooperative adaptive cruise control. If appropriate policies are implemented by all highly motorised nations with the next five years, the anticipated benefits to congestion, accident rates, carbon dioxide emissions and journey time reliability could materialise by 2035,' says Fenner.

For more information contact the ICE Proceedings editor Simon Fullalove on +44 (0)20 7665 2448, email