Slightly difficult because if you try to cut back leylandii, the growth is usually at the end tips of the branches with the inner wood dying back if it is cut. If you only want to trim one side of the hedge you could consider completely removing all the lower branches back to the main truck below roof height of your car which would improve manoeuvring visibility. However, this will possibly look rather odd, so you may then feel obliged to remove all the matching lower branches on the other side of the hedge. The problem withleylandii is that nobody really thinks ahead when they plant them, trying to envisage what they will be like in a few years' time because they grow so rapidly and are difficult to prune back in width rather than height. The only other option would be to remove them completely, have the roots dug out and replant a different type of hedge which does not become so intrusive. Beech is far more amenable, as it mostly retains its leaves in winter, so gives privacy without rapidly growing out of control and needing regular pruning.