Instead, power lines are built into the surface of the road.

This could be a future solution for long-distance trucks and buses running on electricity.

Volvo Group says it already has extensive knowledge about electric drive-trains, but in order to become world-leading in sustainable transport solutions, it must find even more solutions that allow the vehicles to operate on renewable energy.

A great deal of this energy will be distributed as electricity, but the challenge is all about supplying the vehicle with electricity power when needed.

"In city traffic, there are currently various solutions and we are researching many others. We have field tests in progress where our plug-in buses are equipped with a battery that can be charged quickly when the buses are at bus stops," said Mats Alaküla, the Volvo Group's expert on electric vehicles and Professor at Lund University.

But for long-distance trucks this will not work since they stop infrequently; to cope with this task they would need so many batteries that there would be no room for any cargo.

Instead, a solution is required where power is continuously supplied to the truck from an external source.

Last year, Volvo built a 400-metre long track at its testing facility in Hällered outside Gothenburg and the company has been testing the system since last autumn.

The method currently being developed and tested by the Volvo Group, together with Alstom, entails two power lines built into the surface of the road along the entire length of the road.

A current collector in contact with the power lines will be located on the truck.

The two power rails/lines run along the road's entire length.

One is a positive pole, and the other is used to return the current.

The lines are sectioned so that live current is only delivered to a collector mounted at the rear of, or under, the truck if an appropriate signal is detected.

As an additional safety measure, the current flows only when the vehicle is moving at speeds greater than 60 km/h (37 mph).

"With this method, electric vehicles could be continuously supplied with power without carrying large batteries," said Mats Alaküla.

"The power line will be built in sections and one section is only live as the truck passes.

"A lot of years remain before this is on our roads, but, if we are to succeed in creating sustainable transport systems, we must invest significantly in research now.

"I am convinced that we will find a cost-efficient way to supply electricity to vehicles in long-distance traffic and we have already come a long way in our research."