Stuttgart, Germany - It is a common scene wherever today's pioneering electric vehicles whirr through inner city streets - startled pedestrians leap aside and cyclists start to wobble.
For electric cars are not only clean, but virtually silent too and there's the rub. The noise of a conventional car engine warns people that a vehicle is approaching. Pure electric cars and diesel electrics are powered by electric motor at low speeds or in traffic, making it harder to hear them coming.
Silent running makes for a calmer panorama in the city but critics see pedestrian safety at risk. Several manufacturers have come up with innovative, technical solutions to the problem.