Someday, we may all be used to silent cars, but currently a person's brain processes a silent car as turned off and no danger. I had my first electric car in the mid '70's - one my dad and I built. After several incidents of pedestrians stepping in front of the car - just as I was about to start accelerating, we added a small buzzer to act as "engine noise" whenever the car was armed and dangerous. It was a simple fix and it worked. There may be other solutions, but this one does not require reprogramming people.