Roadway cross sections include the elements shown below.

Travel Lanes

Historically, 10-foot lanes were standard for "first-class" paved highways. Today, public agencies prefer lane widths of 12 feet for designing freeways and major traffic arterials. For two-lane highways, a 24-foot wide roadway is necessary for buses and commercial vehicles to have sufficient clearance. As demonstrated in the module on Capacity and Level of Service, lane width affects highway capacity. Anything less than 12 feet tends to reduce speed. However, there are instances when existing rights of way and development will control lane widths. These situations must be carefully evaluated in order to develop the safest design. The number of lanes is determined by estimates of traffic volumes and lane capacity, as discussed in the Capacity and Level of Service module.

Slopes usually fall in both directions from the centerline of two-lane highways. Each half of a divided roadway is sloped individually and may be crowned separately as well. Drivers barely perceive cross slopes up to 2 percent; 1.5 to 2 percent are common cross slope values. Values greater than 2 percent can be unsafe.

Shoulders

The shoulder is the portion of the roadway between the outer edge of the traveled lane and the inside edge of the ditch, gutter, curb, slope or median (in divided roadways). As drivers, we all know the benefits of having adequate shoulder widths when our cars break down. Shoulders also provide lateral support for pavement subbase, base and surface courses. Shoulder widths are usually determined by the traffic volume and the percent of heavy vehicles. Shoulders vary in width from 2 feet to 6 feet on non-freeway roadways and from 4 feet to 10 feet on freeways or other major roads. Shoulders are sloped so that fluids drain away from the traveled roadway. In general, asphalt or concrete-paved shoulders are sloped from 2 to 6 percent, gravel shoulders from 4 to 6 percent and turf shoulders at about 8 percent.

Sideslopes