Imagine how many trucks/vans/cars stay of the road. Imagine the reduced Pollution and noise because of that.
But it doesn't matter how many of them there are - they could be able to fly at an altitude where they don't cause meaningful noise pollution.
For context, the global average cost for electricity is about $0.13 USD per kWH. From wikipedia, "Over an 8-hour work shift, an average, healthy, well-fed and motivated manual laborer may sustain an output of around 75 watts of power." [0] So, from a purely mechanical perspective, a human can only output about 0.6 kWH of work per day, which is only worth about $0.08 of electricity per day. And yet, in states like California, where this test is being done, human labor costs a minimum of $15/hour, or $120/day.
Energy is not the limiting factor. We could use drastically more energy and still save gobs of money if we can cut the cost of employing a human out of the loop.
Say someone drives a 200 horsepower car. That's roughly 1200 kWh/day of available mechanical power.