I think it could work too, Neal, but there are technical considerations to
overcome. If, for example, there were four cars, three from your right and
one from your left, the amount of beeps could become so tangled that you
would not be sure what was happening, just that there were cars. We can
discriminate by ear when we hear car sounds, but if the beeps all sounded
the same, it would sound like a geiger counter orgy.
The cheapest solution is for the cars to make sound, but that probably will not fly with the manufacturers nor the customers. Not that I really care about whether they like it or not, but such concerns will drive the issue and marketplace.