Are vehicles driven in electric mode so quiet that they need acoustic warning signals

Abstract

It has been suggested recently that vehicles, driven in electric mode, either hybrid or pure electric vehicles, are so quiet that they constitute a safety hazard for pedestrians and bicyclists in traffic. It is claimed that such vehicles are not acoustically perceived due to the power unit being exchanged from a combustion engine to electric motors; something that essentially cuts away all power unit and leaves tyre/road noise, the latter of which is the same as for similarsized vehicles with combustion engines. There are currently a number of fast and concerted actions by the US and Japanese governments as well as within international bodies such as UN/ECE and ISO, with the expected outcome that minimum noise of vehicles shall be measured with a standard method and legal limit values for such minimum noise shall be established. The paper present findings regarding possible traffic safety effects of quiet vehicles and concludes that only a US study has identified such effects. A critical review leads to the conclusion that this study may be biased and needs confirmation by further research. After reviewing data from measurements in Japan, the authors present own previously unpublished data on emission levels for road vehicles which may be considered as quiet. Special concern is given to at speeds below 20 km/h where it is expected that the problem might be the worst and where previous data are missing. It is concluded that already a significant number of our present internal combustion engine vehicles are so quiet at low speeds that normally one cannot hear any difference between an electric and a normal vehicle in an urban area. Tyre/road is the dominating in most cases where a light vehicle is driven at speeds at or above 15-20 km/h (heavy accelerations are the exceptions), and this is the same whether the vehicle is electric or not. Thus, it is a property of our vehicle fleet which we have had for more than a decade, and few have considered that as a safety problem. Therefore, there is not enough justification for equipping our future quiet vehicles with extra artificial or warning sounds. If needed at all, there are better options which are non-acoustical.