Although the idea has some merit funds for properly maintaining even the present highway systems are very scarce. It is unlikely a huge expansion of the road system would gain support in a society that will not even raise enough taxes for normal necessary road maintenance.

Most good cities have a pedestrian path system that could also be used by cyclists, which kids should be traveling on.

This sounds like great fun and in fact could be an energy savings and an early teacher for technical know-how.

Unfortunately, I'm sure you son also understands the drawbacks and the reasons why it isn't done. But the idea of a separate set of roads for light, slow vehicles may still be viable in some urban and suburban areas. I know that bike-paths following former railroad lines or creeks are very popular in my area, though they're seldom used (or useful) for transportation. Just recreation.

Maybe a park with an extensive set of go-cart paths that children could drive on would help to satisfy his longing, even though it wouldn't produce the practical results he's trying for.

I have two teen-age drivers that have allowed me to discover how much easier life is when I'm not depositing them here-and-there. But it also means we spend less time together. I've also discovered that they're very navigationally challenged--there are places ten blocks away that they can get lost going to.