> A friend of mine from the US told me that one of the most surprising things about visiting Shenzhen was how silent the downtown was, because almost all the cars are electric there

Can that be true?

I've read at some point at which speed on average the sound of tires on tarmac will be louder than the sound of an engine, and it was a surprisingly low speed. I don't recall the exact number, but I remember that it was below the average in-city traffic speed (save stop-and-go averages).

This definitely matches my real-world experience. I can't usually make out engine sounds in passing traffic, because the tire noise is so freaking loud. I certainly couldn't blindly tell electric from ICE by sound of a passing vehicle at, say, 40 or 50 km/h.