Up to the legal speed limit of whatever road they're on.
i would think not
If your on public roads you can drive at any speed up to the speed limit for the road your on provided it is safe to do so. In the UK the fastest speed limit is 70 mph on most motorways and duel...
By yourself, yes it is.
It depends on the State and Country. Consult your local RMV or Driver's Handbook for the answer.