Road & Highway Cross-Sections: Design and Components
Road and Highway Cross-Sections
Introduction
The cross-section of a road or path describes its geometric characteristics as a plane perpendicular to the vertical surface containing the road’s axis. This cross-section varies along the road due to the combination of different elements, whose sizes, shapes, and relationships depend on their functions, the nature of the track, and the surrounding terrain.
Platform
The platform is the visible surface of a pathway, including the driveway(s), berm(s), platform widening (SAP), and median (if present). The platform width is the sum of the widths of these elements. Special cases include roads with separate lanes and unpaved roads.
Roadway
A roadway is a defined band of material designed to withstand vehicular traffic and allow safe and comfortable travel. It consists of two or more lanes, each accommodating a row of vehicles moving in one direction. On two-way roads, each lane may be used occasionally by vehicles overtaking in the opposite direction.
Median
Roads with four or more paved lanes typically have a median, a space between the inner edges of the pavements. One-way roads with independent platforms may have varying distances between their axes, resulting in an uneven median.
Roadway and Platform Widths
Except for local roads and development projects with speeds less than or equal to 60 km/h, the minimum lane width is 3.5 m. The berm, SAP, and median widths depend on the project level and speed. The selection of the cross-section depends on the project’s function, terrain type, and traffic study predictions.
Slow Traffic Lanes
On two-way roads, slow traffic lanes are widened on the right side. On one-way highways, they are widened on the median side. Auxiliary lanes on two-way roads should be long enough to allow vehicles to exit safely. If this is not feasible, sections of 500 m with overtaking possibilities should be provided.
Fast Tracks
Fast tracks are marked on the median side with vertical signs. The increased width should be over 60 meters for speeds below 70 km/h and 80 meters for speeds above 80 km/h.
Increase and Decrease in Number of Lanes
Increases in the number of lanes should be made over 60 m (for speeds below 70 km/h) or 80 m (for speeds above 80 km/h). Decreases should be conducted over 150 m (for speeds below 70 km/h) or 200 m (for speeds above 80 km/h).
Cornering Widening
Roads may require widening at curves with small radii.
Cross Slope (Pumping)
Roadways should have a minimum cross slope or pumping to evacuate surface water. This slope depends on the surface type and rainfall intensity.
Bidirectional Pumping
This can be achieved with a high point in the center of the roadway or with one edge higher than the other. The latter is preferred for future expansion to a one-way road.
Unidirectional Pumping
The camber axis may be located in the middle of the road or on the inner edges of the pavements.
Berms
Berms are the stripes flanking the roadway(s). They protect the pavement, allow occasional stops, provide a clear lateral view for drivers, and offer space for emergency maneuvers.
Berm Widths
Berm widths vary depending on the road type and speed, ranging from 0.5 m to 2.5 m.
Transversal Slope of the Berms
On paved roads, berms have the same cross slope as the roadway. On unpaved roads, they maintain the cross slope of the road.
Platform Widening (SAP)
SAP provides confinement for the subbase and base layers. The minimum width is 0.5 m. If safety barriers are installed, the minimum width is 0.8 m.
The Median
The median is the space between the inner edges of the roadways. It enhances safety, controls glare (with plantings), and provides space for left turns at intersections.
Median Widths
The minimum median width varies depending on the design speed and road category.
Occasional Additional Tracks
Bus Stops
Bus stops should be located in areas with good visibility and should not hinder visibility at intersections.
Brake Beds
Brake beds are designed to stop vehicles with brake failures on long, steep slopes.
Bikeways
Bikeways are provided on collector and local roads with speeds up to 70 km/h when bicycle traffic exceeds a certain threshold.