I'm not going to comment on how well this device would actually work in practice. I would however like to point out that it is largely believed to be illegal to ride a bike on the sidewalk (and in most places, it's true). San Francisco comes to mind, for example. It's illegal where I live as well, although it's rather selectively enforced.

As a cyclist, I can unfortunately attest that the majority of street riders are indeed idiots. To be fair however, that's just as true for motorists. ;)