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With the emergence of EVs, a reduction in road traffic noise by a few decibels is anticipated at least for low-speed situations, thus the potential for some reduction in adverse health outcomes (Campello-Vicente et al., 2017). There is an even greater potential for noise reduction in areas with a large number of powered two-wheel vehicles (e.g., motorcycles, motor scooters), such as in southern Europe. In these areas, replacement of powered two wheelers by electric ones could substantially reduce noise levels and noise annoyance as shown in Figure 1 (Fiebig et al., 2012).
However, associations of the blind and visually impaired have pointed out that EVs pose a growing threat to all pedestrians, particularly to those who depend on hearing (Pierce, 2007). For example, the European Blind Union (EBU; 2019) advocated for the addition of mandatory sounds in all silent cars. The media picked up this topic and reported on the growing hazard due to “near-silent electric vehicles” (Birch, 2009) and on EVs as “silent killers” (Okulski, 2012).
Early studies investigating the danger of EVs to the visually impaired, based on the analysis of accident statistics, suggested a higher risk of collisions (US National Highway Traffic Safety Administration [NHTSA], 2009). This led to political action and increased efforts to develop adequate regulations. In particular, associations of the blind emphasized that visually impaired pedestrians require acoustic input to be aware of traffic to safely cross a street; otherwise they face enormous safety issues (EBU, 2019).
Early studies on the effect of artificially generated sound emitted by EVs showed, as expected, that an EV with artificially added sound will be detected at a significantly greater distance than an EV without artificially added sound (Kim et al., 2012). It became clear from the earliest research that certain noise characteristics, such as those from emergency alarms, animal sounds, and melodious sounds, are inappropriate for EVs because they could confuse identification of the source and/or that they could produce adverse human responses (United Nations Economic Commission for Europe [UNECE], 2012).
Nevertheless, it took several years until the idea of using alert signals and defining their characteristics for improved detectability of EVs was put into action. It also took time before (new) EVs were required to comply with regulations specifying alert sound requirements. Yet, there
is still a debate about how pedestrians can be alerted by appropriate sound signals and how road traffic noise can be reduced through electrification of the powertrain.
The Starting Point
In 2009, the NHTSA published a technical report comparing the incidence rates of pedestrian and bicyclist crashes involving EVs to the incidence rates with ICE vehicles under similar circumstances. The report found an increased risk of accidents for pedestrians and cyclists concerning collisions with EVs compared with ICEs (NHTSA, 2009). Around the same time, the UNECE World Forum Working Party 29 recognized that the positive environmental benefits achieved by EVs brought with it the conflict that the reduced audibility of vehicles presented a danger to pedestrians. Thus, the use of acoustic means was proposed by the working group “Quiet Road Transport Vehicles” (QRTV; 2010) to mitigate potential pedestrian hazards.
The NHTSA publication (2009) about accident statistics triggered a substantial debate. For example, Sandberg et al. (2010) strongly questioned the conclusions drawn from meta-analyses of accident statistics and indicated that some details in the NHTSA report (2009) are arguable or at the very least unclear (e.g., consumer bias, no consideration of vehicle kilometers driven). A study investigating the accident risk posed by EVs compared with equivalent ICE vehicles in Great Britain for the years 2005 to 2008 observed that EVs were equal or less likely to be involved in collisions with pedestrians than ICE vehicles, but the authors questioned the validity of this outcome (Morgan et al., 2011). In 2011, an update of the NHTSA study showed similar trends to those in the 2009 report and again found higher incidence rates for EV versus ICE vehicles while paying special attention to the statistical power with a three times larger EV sample size (Wu et al., 2011).
Although the debate about the benefits and drawbacks of warning signals for EVs continues, regulations on minimum noise requirements for electric cars have been implemented on national and international levels.
The Actions
In 2010, the US Pedestrian Safety Enhancement Act (PSEA) was introduced and it became law in 2011 (PSEA, 2011). This act asked for rulemaking by determining
Winter 2020 • Acoustics Today 21