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100 additional 4 feet, or a total shoulder width of 10 feet, is required to permit and encourage drivers of disabled vehicles to stop clear of the traffic lanes so as not to constitute a traffic hazard.

Effect of vehicle length and off-tracking on curves

When a vehicle makes a turn, the rear wheels follow a path having a shorter radius of curvature than the path followed by the front wheels. The distance between the two paths is known as the amount of off-tracking. As a result of off-tracking, vehicles negotiating horizontal curves occupy a greater width of roadway than they do in following a straight course.

The amount of off-tracking increases with the length of vehicle and varies inversely with the radius of the curve. The off-tracking of passenger cars is insignificant on modern highways except at intersections and on the ramps at interchanges. The off-tracking of trucks and truck-trailer combinations, however, is much greater than for passenger cars and governs the amount by which the normal lane widths must be widened on the sharper curves. In some cases, it establishes the minimum curvature which it is practical to employ for the design of intersections at grade and at interchanges. The off-tracking of the larger vehicles that are likely to use the interstate highway system is not sufficient to require lane widths in excess of 12 feet on any curve up to 9 degrees, which is the absolute maximum permitted in the design standards for the system. It is significant that on the sharper curvatures which may be employed in the vicinity of intersections and interchanges, the 35 feet for single-unit trucks, the 45 feet for tractor-semitrailer combinations, and the 60 feet for truck-trailer combinations proposed as limits of lengths by the American Association of State Highway Officials, off-track approximately the same amounts on correspondingly sharp curves. The off-tracking of these vehicles varies from about 0.8 foot on a curve with 500-foot radius to about 7 feet on a 50-foot radius.

SPEED AND NUMBER OF TRUCKS DETERMINE GRADE LENGTHS AND HIGHWAY CAPACITY

Commercial vehicles occupy a greater road space and influence other traffic over a larger area of highway than do passenger cars because they are larger and generally travel at lower speeds, especially on grades. When a highway is operating at its capacity the total number of vehicles is therefore less, if there are any commercial vehicles, than if traffic is composed entirely of passenger cars.

Effect of trucks on capacity

In relation to highway capacity, one commercial vehicle has approximately the same effect as two passenger cars in level terrain and four passenger cars in rolling terrain. In mountainous terrain the effect of one commercial vehicle may be as great as eight passenger cars. On individual grades trucks affect the safe and efficient flow of traffic because they go so slowly upgrade that long queues of passenger cars are formed behind them and then go so fast down the other side that none can pass. This type of congestion tends to cause the drivers of passenger cars to take risks in passing the trucks at points where sight distances are inadequate.