51. Liability for obstruction.

The placing1 of an obstacle on a highway or such an improper use of a highway as amounts to an obstruction of the traffic on it constitutes a nuisance at common law2. In some cases obstruction of the highway may be authorised by law3 or agreement4. It may be either temporary5 or permanent6. Where it is temporary, there is a duty, breach of which is an act of negligence, to remove the obstruction as soon as possible7, and to see that it is so guarded that the risk to persons using the highway is reduced as far as possible