357. On a one-way roadway, the driver of a road vehicle who is about to turn left at the intersection of a two-way roadway must, after signalling his intention and ascertaining that he can do so in safety, move to the extreme left of the roadway on which he is driving, proceed in a straight line to the near side of the roadway he is about to enter and turn left as soon as the way is clear, so as to enter the right-hand part of the other roadway.