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.

Of course the effortless torque of a near-silent engine is one of the most appealing aspects of the electric car, said Frank Weber, who developed the Chevrolet Volt for US carmaker General Motors.

"The powerful 150 horsepower electric motors really make the Volt fly and the lack of engine noise is a real gain. We've finally arrived at where we have been trying to get to with the internal combustion motor for decades," said Weber.

Residents used to putting up with incessant traffic noise are breathing a sigh of relief and testers have universally praised the silent drive. It has been compared with soaring aloft in a glider or travelling on board an ocean-going yacht.

At the same time, the growing number of diesel-electric registrations is worrying representatives of organisations for the blind and partially-sighted. They point out that the lives of visually-impaired pedestrians, who take their cues on whether to cross the road from the sound of oncoming cars, could be put at risk by soundless vehicles.

"For that reason we believe there is scope for further developing these vehicles so that blind and partially-sighted people can recognise them," said Clive Wood, transport policy officer at the Guide Dogs for the Blind Association in Britain.

Some manufacturers have come up with artificial sounds which mimic real engines. In order to satisfy the desire of customers for the brand's typical throaty engine note, British sports car maker Lotus from Hethel near Norwich has unveiled its "Safe & Sound" hybrid based on the diesel-electric Toyota Prius.

The system used by Lotus consists of a waterproof loudspeaker behind the front grille which projects the sound of a traditional engine, depending on speed and throttle, when the Prius is running in electric mode.

The company says the synthesized sound is only detectable from the front of the vehicle, where it serves to warn pedestrians. The driver cannot hear any of the additional noise.

Using the system engineers can wire up even small cars to sound like a V8-powered limousine but the current emphasis is on retaining the positive environmental aspects of quieter cars.

German motor components supplier Eberspaecher in Esslingen near Stuttgart offers a similar system called "ActiveSilence." It neutralises unwanted road surface and noises by using near identical sound waves to cancel them out.

It can enhance sounds too so that a beefy V8 engine can still burble when its active fuel management system switches off half the cylinders under light load to conserve fuel.

The device responds to the flick of a switch, enabling the driver to choose between a refined or more sporting engine note. It can be fitted into any exhaust system but will not work in purely electric cars.

Even the tuning companies have not turned a blind eye to potential of customising engine sounds. Brabus in the Ruhr district town of Bottrop has developed what it calls a "Space Sound Generator" for the electrically-powered Tesla Roadster. The driver can select a range of artificial engine noises, including the sound of a Formula One racing machine in full cry.

Another group of experts has meanwhile come up with a wrist-held vibration alarm similar to those used in cell phones. Radio waves beamed from an electric vehicle cause the device to start throbbing and warn a visually-impaired person that a car is approaching.

Chevrolet Volt developer Frank Weber has tackled the problem from a different angle: "We have built in a few extra safety features too. The Volt has a friendly-sounding pedestrian warning alert which is more pleasing than a horn and you can activate it much like you would your high-beam." (dpa)