(a) All commercial tractors, trailers, semi- trailers and pole trailers and trucks in excess of 7,000 pounds gross weight shall be operated in the lane nearest the right-hand edge of the roadway upon all through streets when parking is prohibited in such traffic lane; otherwise, in the lane of traffic immediately left thereof. No such vehicles shall overtake and pass any other moving vehicle traveling in the same direction in the right-hand lane of any through street unless otherwise directed by a police officer or traffic-control device.
(b) The provisions of this section shall not apply when such vehicles are making an approach for a left turn or when the lane nearest the right-hand edge of the roadway or the lane immediately left thereof upon any through street is not available for travel, or when the lane nearest the right-hand edge of the roadway is reserved for vehicles making or tending to make a right turn at the next nearest intersection and when traffic-control devices have been placed or erected to so indicate the restricted use of such traffic lane.