About Surface Elements

All linear constructions such as roads depend on alignment, which is the base common infrastructure for the road surface design. This topic provides information about the road surface structure.

Roadway: The portion of a highway, including shoulders, for vehicular use. A divided highway has two or more roadways.

Traffic lane: portion of the roadway.

Traveled way: The portion of the roadway for the movement of vehicles, exclusive of shoulders and bicycle lanes.

Shoulder: The portion of the roadway contiguous with the traveled way that accommodates stopped vehicles, emergency use, and lateral support sub-base, base, and surface courses.

Turnout: Provides an area for emergency stops and allows slower moving vehicles to pull out of the through lane to permit following vehicles to pass.

Traffic Median: The area that separates opposing lanes of traffic on divided roadways. Lanes and medians can be distributed on each side of a planar or crowned surface.

Traffic Island: A solid or painted object in a road that channels traffic. It can also be a narrow strip of island between roads that intersect.

Shoulder profile can be:

  • Planar, introducing a sharp grade break between shoulder and traveled way.
  • Convex, providing a smoother transition between shoulder and traveled way.
  • Or planar with multiple breaks, getting closer to convex but limiting the construction complexity.

Roll-Over: Algebraic difference between traveled way and shoulder. The maximum value should be from 6 to 7%.

General widening: refers to the change of traveled way width consecutive to the addition/removal of a turnout, traffic lane, median or island or to the proximity of an obstacle.

Curve widening: refers to the additional width of traveled way that is required on a curved section. The amount of curve widening of the traveled way on a horizontal curve is the difference between the width needed on the curve and the width used on a tangent.Shoulders are traditionally not widen in curve sections since vehicles are not supposed to travel on them. If a shoulder lane is specified with a design vehicle and speed then curve widening is computed as it is for traveled way.

Curve widening is composed of mechanical and psychological widening:

  • Mechanical widening: When a vehicle negotiates a horizontal curve, the rear wheels follow a path of shorter radius than the front wheels. This phenomenon is called off tracking, and has the effect of increasing the effective width of a road space required by the vehicle. Therefore, to provide the same clearance between vehicles travelling in opposite direction on curved roads as is provided on straight sections, there must be curve width of traveled way available.
  • Psychological widening: The widening of the traveled way has also to be done for some psychological reasons. Drivers tend to drive close to the edges of the pavement on curves. You need to provide some curve space for more clearance for the crossing and overtaking operations on curves.