Drivers should drive on the right half of the road (with some exceptions). These exceptions include:
Passing another vehicle moving in the same direction on a road where passing is permitted.
Driving around an obstacle or blockage that makes it necessary to drive to the left of the center line. Make sure to yield to oncoming traffic, though.
Drivers are permitted to drive on the left half of the road when a road is divided into three marked lanes for traffic; when a one-way street has two or more lanes; when crossing the center line to make a left turn; or when approaching a stationary emergency vehicle.
If you are driving on an interstate highway or an full access-controlled freeway or tollway and traffic is not congested, the left lane should be used only as a passing lane.
Keeping the left lane open as a passing lane allows faster traffic to pass slower-moving traffic.
If traffic in all lanes on an interstate highway, freeway, or tollway is moving slowly, drivers are allowed to drive in any lane, including the left lane.