You may drive on the left side of a multilane highway only when you are

Driving on the left side is legal only in some cases, such as on one-way streets and while passing. If you are driving at about the same speed as other traffic, stay in the middle lanes. Leave the leftmost lane free for passing and the rightmost lane free for vehicles entering or leaving the highway. If you are moving slower than the posted speed limit, drive in the rightmost lane unless you are passing, turning left, or avoiding an obstruction.