Traffic lanes are about 12 feet wide. From a motorcyclist's perspective, the lane is divided into three sections each four feet wide: left, middle and right. The following illustrates these sections. The middle third is that meaty third of the roadway generally designated by an oil slick created by years of dripping fluids from passing vehicles.
There is some debate about lane positioning among the experts. Some say that, all things being equal, a motorcyclist should ride in the left third. Others advocate riding in the middle third just left of the oil slick. But there is agreement that a rider should keep to the left when given a choice.
There are many reasons for riding on the left, whether in the middle of the left third of the lane, or left of the middle in the middle third of the lane. First, this is where you can see the most. Second, this is where you can most easily be seen. Third, this offers you the greatest number of avenues for escape since you have paved areas to both your left and right. Fourth, this is where you have the most time to react to hazards coming from either curb.
But you will not always ride in the left position. You will move from the left position to set up a turn, view something up the road that represents a potential hazard, let another driver see you or avoid a hazard. For example, a car driver entering from the right may not notice you behind a van. If you can make yourself known to the car driver by moving right for a brief period, do so. Or, if an oncoming Eighteen-wheeler is plowing a huge wake of wind, you can avoid the worst of the blast by moving right.
Respect, but do not fear the oil slick. Dangerous oil buildup is rare. In dry weather, the slick itself provides sufficient traction and can be ridden on safely in most instances.
Significant oil and grease accumulations occur where traffic stops, such as at intersections and toll booths. Be extremely cautious at toll booths! Often large, deep lakes of oil can be seen. The oil slick is also wide, virtually the entire width of the lane. When stopping to pay the toll, you may not find the traction you expect when you put your foot down. When you leave the booth, drive away slowly and cautiously. You may find that the tires have received a coating of oil and do not grip as well as they should. You may also find that your soles are coated with oil and do not grip the footpegs or road surface well.