It is better to contact the RSPCA, SSPCA, USPCA or a reputable swan rescue agency, such as The Swan Sanctuary in Shepperton, than to attempt to handle a distressed swan yourself.
Swans occasionally land on busy carriageways. It is thought that thermals and wind turbulence over roads may knock the swans off balance and bring them down onto road surfaces. They need a long run before taking flight, and may be unable to take off if there is insufficient space.
If the bird is found on the road or trapped on the road, or a rescue represents a real danger to the rescuer, the police should be informed.
Swans are very attentive parents, so a young lone cygnet may mean that something is amiss. Young cygnets are very vulnerable, so if you find one that you are absolutely sure is orphaned, put it safely into a cardboard box that contains a clean cloth.
You should then contact a reputable swan rescue agency, e.g. The Shepperton Swan Sanctuary, who will give you the necessary advice on how to care for it until they can collect it. You should not attempt to hand-rear the cygnet yourself.