At parking-lot speed, why do quiet EVs need alert sounds before tire noise helps?

Answer: Tires are not loud yet

Tires are not loud yet ✓ — Right. At low speeds, electric and hybrid vehicles can be much quieter than combustion cars because tire and wind noise have not yet taken over. That is why U.S. rules require warning sound up to about 30 km/h, and why the sound is not required at higher speeds where tire and wind noise provide enough warning. The mechanism is speed-dependent masking. A car can be audible on the highway and still too quiet in a parking lot.

Wind noise is enough — Not quite. Wind noise grows with speed, so it can help make vehicles audible later. At parking-lot speed, though, wind and tire noise have not yet taken over from propulsion noise. That is why alert-sound rules focus on low-speed operation. The safety gap is exactly the quiet zone before natural road and air noise become useful.

Silence is safer — Not quite. Quiet vehicles can be pleasant, but silence is not automatically safer for people outside the car. Low-speed alert rules exist because pedestrians may not notice an approaching electric or hybrid vehicle soon enough. A parking-lot cue has to be detectable, not merely peaceful. The alert sound replaces the accidental warning that combustion engines once provided.