Researchers have come up with a more eco-friendly idea for satnavs.

Rather than routing cars on the shortest route, the device routes drivers to roads on which they can travel at the most constant, slow speed.

“The most efficient roads are often those where you can drive at a fairly slow, constant speed, without having to stop or suddenly accelerate or decelerate”, research Dr Eva Ericsson said to the New Scientist.

The device checks roads for width, speed limits, and traffic flow during rush hour.

The device is being developed and researched by scientists at the Lund Institute of Technology in Sweden, who checked data from over 15,000 trips around Lund to rank roads based on efficiency.

The scientists say their research indicates that using the green satnav could reduce fuel consumption be 8.2%.

They're still trying to work out kinks in the system, like the fact that they haven't been able to detect or identify as many traffic jams as they need to make the device effective.