Superelevation

Superelevation is the rotation of the pavement on the approach to and through a horizontal curve. Superelevation is intended to assist the driver by counteracting the lateral acceleration produced by tracking the curve. Superelevation is expressed as a decimal, representing the ratio of the pavement slope to width, ranging from 0 to 0.12 foot/feet. The adopted criteria allow for the use of maximum superelevation rates from 0.04 to 0.12. Maximum superelevation rates for design are established by policy by each State.

Selection of a maximum superelevation rate is based on several variables, such as climate, terrain, highway location (urban vs. rural), and frequency of very slow-moving vehicles.  For example, northern States that experience ice and snow conditions may establish lower maximums for superelevation than States that do not experience these conditions.  Use of lower maximum superelevation rates by policy is intended to address the perceived problem created by vehicles sliding transversely when traveling at very low speeds when weather conditions are poor.

The adopted criteria provide complete tables expressing the appropriate superelevation rate consistent with the established policy for all curves and all design speeds.

Figure 15 is a photo showing oncoming cars rounding a curve in the road.  The photo shows how the roadway is banked, or superelevated.

Clarifications

A formal design exception is required if the State’s superelevation policy cannot be met in design of any curve on the NHS.  Thus, if a State’s maximum policy is set at 0.06 and a design is proposed that would use a superelevation rate greater than 0.06 (but within overall AASHTO guidance) this is considered an exception. A design exception is also required if a superelevation rate is proposed that is different from the published rate per the State’s policy for that curve, regardless of whether the curve is a controlling one (minimum radius for a design speed) or not.

Note that no design exception is required for superelevation transition lengths.  Also, some States employ spiral curves for high speed and sharper curves to help develop superelevation. For States that use spiral transitions, the inability or decision to not use a spiral does not require a design exception.

Safety and Operational Considerations

The safety and operational concerns related to inadequate superelevation are similar to those discussed in the horizontal alignment section.  Inadequate superelevation can cause vehicles to skid as they travel through a curve, potentially resulting in a run-off-road crash.  Trucks and other large vehicles with high centers of mass are more likely to roll over at curves with inadequate superelevation.

Substantive Safety

Table 12 reports how variations in superelevation affect safety on rural two-lane highways. A superelevation deficiency is one in which there is insufficient superelevation compared to that specified by the appropriate design policy and values.

Source:  Prediction of the Expected Safety Performance of Rural Two-Lane Highways, FHWA

Summary

Table 13 summarizes the potential adverse impacts to safety and operations of a design exception for superelevation.

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.

Superelevation Resources

• Low-Cost Treatments for Horizontal Curve Safety, FHWA, 2006.
  
  
• 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 Involving Heavy Trucks, NCHRP Report 500, Volume 13, Transportation Research Board, 2004.
  
  
• Review of Truck Characteristics as Factors in Roadway Design, NCHRP Report 505, Transportation Research Board, 2003.
  
  
• A Guide for Addressing Head-On Collisions, NCHRP Report 500, Volume 4, Transportation Research Board, 2003.
  
  
• A Guide for Addressing Run-Off-Road Collisions, NCHRP Report 500, Volume 6, Transportation Research Board, 2003.
  
  
• Roadside Design Guide, AASHTO, 2002.
  
  
• Guidelines for Geometric Design of Very Low-Volume Local Roads (ADT ≤ 400), AASHTO, 2001.
  
  
• FHWA Roadside Hardware Web site, http://safety.fhwa.dot.gov/roadway_dept/policy_guide/road_hardware/
  
  

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