When silence is dangerous: e-scooters and sound

The pursuit of alert sounds for e-scooters

If you live in a city, you’ve probably shared a sidewalk with an electric scooter. Or maybe you’ve been startled by an e-scooter zooming past you from behind, and you didn’t hear it coming. Maybe you thought “These scooters are so quiet that they’re dangerous.”

And you’re not wrong. When electric cars first rolled out, many of them were also silent. Compared to gasoline-powered cars, electric vehicles are 25% more likely to get in accidents or close calls with pedestrians if they don’t emit an alert sound. In Europe, electric cars are required to produce a tone between 160 Hz and 5 kHz when moving at a speed “greater than 0 km/h and up to and inclusive of 20 km/h” (12.4 miles per hour) to alert pedestrians of a moving vehicle. At the same time, the sonic alert sound should not create an annoyance.

However, electric scooters don’t produce a warning sound, even though they often travel on the same sidewalk with pedestrians. The pedestrians might not hear a scooter coming behind them, and therefore don’t have enough reaction time.

Researchers tested e-scooters with and without alert sounds

In a published paper titled “Development of electric scooter alerting sounds using psychoacoustical metrics,” researchers at the Acoustics Research Centre at the University of Salford document how they tested two different environmental noise scenarios to observe how easily an alert sound can be heard in those environments. One was a city park at 50 decibels, and the other was a city street at 60 decibels. The most-effective sound was described as “an impulsive 1 kHz tone at a rate of 7 Hz.”

For reference, a single 1 kHz tone sounds like this:

Researchers found that without an alert tone, pedestrians detected an approaching electric scooter at about 4.8 meters (15.8 feet) in a 50-decibel test environment and just 2.5 meters (8.2 feet) in the 60-decibel environment.

In contrast, the researchers discovered that a 56 dBA sound produced the best results for an e-scooter. It improved pedestrian detectability when the e-scooter traveled at a speed of 20 km/h (12.4 miles per hour). The alert tone increased the mean detectability distance to 30 meters (98.5 feet) in the 50-dBA city park environment. And in the 60-dBA street-noise scenario, detectability increased to 18.2 meters (59.8 feet), a significant improvement over the silent e-scooter.

You can read the research paper in full at Science Direct. The researchers were Tim Walton, Antonio J. Torija, and Andrew S. Elliott.