Cross Slope

Pavement cross slope is an important cross-sectional design element. The cross slope drains water from the roadway laterally and helps minimize ponding of water on the pavement. This prevents maintenance problems and also minimizes icing from occurring on poorly drained pavement.   On roadways with curbed cross sections, the cross slope moves water to a narrower channel adjacent to the curb, away from the travel lanes, where it can be removed. Cross slopes that are too steep can cause vehicles to drift, skid laterally when braking, and become unstable when crossing over the crown to change lanes.  These conditions are exacerbated by icy, snowy, or windy conditions. Both maximum and minimum criteria exist for cross slope. A formal design exception is required wherever either cannot be met.

Clarifications

Cross slope criteria apply to typical tangent alignments. On high-speed roadways, normal cross slope is 1.5–2.0 percent, with the cross-slope break (the algebraic difference in slopes between the lanes) at the centerline not exceeding 4 percent. In areas of intense rainfall and where there are three or more lanes in each direction, additional cross slope may be necessary for adequate drainage. Accomplishing other design features (superelevation transitions, pavement warping at intersections, etc.) will inevitably require removal of cross slope in spot locations. These cases are routine and necessary in design and a design exception is not required.

In addition to the cross slope of the lanes, the cross-slope break on the high side of superelevated curves should not exceed 8 percent (Figure 23).  A formal design exception is required when this condition is not met.

Figure 23 is a photo showing the high-side shoulder along a curve with a cross slope in the opposite direction as the superelevated travel lanes.

Summary

Table 18 summarizes the potential adverse impacts to safety and operations of a design exception for cross slope.

Freeway:  high-speed, multi-lane divided highway with interchange access only (rural or urban).
Expressway:  high-speed, multi-lane divided arterial with interchange and at-grade access (rural or urban).
Rural 2-Lane:  high-speed, undivided rural highway (arterial, collector, or local).
Urban Arterial:  urban arterials with speeds 45 mi/h (70 km/h) or less.

Cross Slope Resources

• A Policy on Design Standards Interstate System, AASHTO, 2005.
  
  
• A Policy on Geometric Design of Highways and Streets, AASHTO, 2004.
  
  
• A Guide for Reducing Collisions on Horizontal Curves, NCHRP Report 500, Volume 7, Transportation Research Board, 2004.
  
  
• A Guide for Reducing Collisions Involving Heavy Trucks, NCHRP Report 500, Volume 13, Transportation Research Board, 2004.
  
  
• A Guide for Addressing Run-Off-Road Collisions, NCHRP Report 500, Volume 6, Transportation Research Board, 2003.
  
  
• Guidelines for Geometric Design of Very Low-Volume Local Roads (ADT ≤ 400), AASHTO, 2001.
  
  
• Highway Drainage Guidelines, AASHTO, 2000.
  
  

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