Volvo has tested its 'road train' principle for the first time on a public road.

A road train is a convoy of semi-autonomous cars, all following a lead car driven by an actual person on a motorway; the following cars automatically keep a set distance between each other.

The SARTRE project (safe road trains for the environment), it's hoped, will see motorways become more free flowing, which will reduce congestion, fuel use and accidents.

The system works using the tech found in radar cruise control, although it's taken a step further because the cars can steer themselves. The advantage of using largely existing technology is that road trains can be implemented without huge costs or motorway infrastructure changes.

The first public test saw three Volvo cars and a truck following the lead driver for 120 miles on the outskirts of Barcelona.

SARTRE project leader Linda Wahlström said: "Driving among other road-users is a great milestone in our project. It was truly thrilling. During our trials on the test circuit we tried out gaps from five to fifteen metres."