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