In another step toward making silent electric cars less so, Toyota Prius hybrids sold in Japan are to be offered with an optional system that broadcasts a "humming" noise outside the car whenever it operates solely on electricity.

The system is Toyota's response to the fears of blind people that hybrids and electric vehicles pose more of a threat to them because they're not as easy to hear.

Pending legislation in the U.S. will require all new hybrids and electric vehicles to fit noise-making equipment, despite a lack of solid evidence proving any risk. In fact, one analysis (and a more rigorous followup) showed a decrease in pedestrian accidents over the period since hybrids were introduced.

The $150 system offered on Japanese-market 2011 Toyota Prius hybrids emits a whirring noise, which the company says is similar to the "synthesized sound of an electric motor." The volume is no higher than that of a combustion-engined vehicle, and the system can be switched off by the driver.

The 2011 Chevrolet Volt range-extended electric car takes a different tack to solve the same problem, providing a pedestrian alert system controlled by the driver, as the horn is. Volt vehicle line director Tony Posawatz described the sound as "a series of low horn audio signals or cues."

Toyota said it will offer the noise-making system at the start of September, and will add the feature to other vehicles that can run in electric-only mode--whether hybrid or fully electric--in years to come.

[Inside Line]