Taken from the Ontario Highway Traffic Act.
Signal for left or right turn
142.(1) The driver or operator of a vehicle upon a highway before turning to the left or right at any intersection or into a private road or driveway or from one lane for traffic to another lane for traffic or to leave the roadway shall first see that the movement can be made in safety, and if the operation of any other vehicle may be affected by the movement shall give a signal plainly visible to the driver or operator of the other vehicle of the intention to make the movement. R.S.O. 1990, c. H.8, s. 142 (1).