England and Wales only: the surface of an adopted highway is the province of the local Highway Authority. The sub-soil generally belongs to the adjoining owner. The HA paid for the tarmac, so even if it breaks up, the detritus remains theirs unless they can be proved to have abandoned it. While it is unlikely (in my ex-professional opinion) that they would want/could afford to prosecute you for theft, or sue you for the cost of replacement, provided you haven't tried to help the breaking-up process along, you should bear in mind
- that the stuff may be contaminated with particulates from vehicle exhausts, oil, petrol and diesel leaks, horse droppings (you mentioned country lanes) and possibly more;
- that you might get hit by a passing motorist as you bend down to sweep;
- that your bicycle may become unstable as a result of uneven loading, rendering you liable to an accident or prosecution;
- the grit may not be the right colour or size after all, or there might not be enough.
To be safe, you ought to seek Counsel's Opinion; but that would buy you enough bags of gravel to last you a lifetime.
Regards,
Mr Justice Cocklecarrot.
"British justice not only has to be done, it has to be seen to be believed."