The center portion of a lane usually contains an oily strip. This part of the lane is:

Oily drippings from cars collect in a strip in the center of a traffic lane. Unless the road is wet, this area will generally still provide enough traction for motorcyclists to ride safely. Because the strip is usually no more than two feet wide, it is often possible to ride to one side of the strip and still be in the center portion of the lane.