Remember kids... always wear a helmet. (The almighty bikeradar helmet thread)

No, it's not. You said you'd encourage cycling with headphones on.

You cannot hear things around you as well with headphones and anyone who cycles in london with them on is nuts.

No it's not, in fact **I'd encourage it **if it force them to be more aware.

As I said, it's an interesting experiment, but it isn't desirable to block a sense. 'If it forced them to be more aware'--what you mean is 'they should look more', but artificially 'forcing' them to do it isn't a good way of doing that. The aim of training should be that they learn to do it without being 'forced'. It shouldn't be 'enocuraged'. It can be used didactically, to demonstrate a point, but not as a constant practice (for hearing people).