I'm pretty sure that depends on the technology. For instance planes just become gliders in the worse possible case.

The bigger issue is when the crash by result of error. There is a limited amount of air space anyway so if you had tons of cars there would need to be highways which would create traffic equivalent to what's on the street.

In short, way more deadly for the same result.

It’s a horrible idea if you assume that flying cars would be made using the technology currently available to us. Imagine what it would be like to own a computer the size of a house. At one point, that was the only kind of computer there was, so it was pretty obvious nobody would want one. Also, the UI was horrible, power consumption was ridiculous, capabilities were very limited etc. If technological development had gotten stuck at that level, computers and the internet would not have become very popular.

However, many things have changed since then, carrying a personal computer in your pocket became possible, many of the old downsides were eliminated, capabilities were expanded, many use cases were invented etc. What was called a computer back then and what we use the word for today are only vaguely related.

Similarly, what we think of as a flying car today, is a complete disaster, because we’re thinking about it in the context of modern technology. In order to make that dream a bit more realistic, we would need to many breakthroughs in many different fields.