one possible option would be to have two cables/chains crossing the road and then tie the two cables together at three or four points (think multiple horizontal "H"s). this way there would be no lifting the top cable/chain to get by.
to prevent liability or at least reduce it drastically, put up signs 50' and 100' from the gate stating "cable across road". then hang reflectors and reflecting tape all over the cables as well as a huge stop sign. in most areas, this would be enough warning to generally prevent liability.
another option, if you don't want to install a full blown gate, would be a single heavy pipe type gate about 3' high. same idea w/ reflectors, etc.
to prevent liability or at least reduce it drastically, put up signs 50' and 100' from the gate stating "cable across road". then hang reflectors and reflecting tape all over the cables as well as a huge stop sign. in most areas, this would be enough warning to generally prevent liability.
another option, if you don't want to install a full blown gate, would be a single heavy pipe type gate about 3' high. same idea w/ reflectors, etc.