Grade

Grade is the rate of change of the vertical alignment. Grade affects vehicle speed and vehicle control, particularly for large trucks.  The adopted criteria express values for both maximum and minimum grade. The inability to meet either a maximum or minimum value may produce operational or safety problems.

A primary safety concern is the potential for drivers of heavy trucks to lose control as they descend steep grades.  A design exception is required if the maximum grade is exceeded.  Minimum grades to achieve proper drainage have also been established, and a design exception is required for highway segments that are flatter than the minimum grade.

Speed differential on highways with steep grades can contribute to safety and operational problems.  Trucks and other heavy vehicles lose speed on steep, ascending grades and may be unable to reach full highway speed until they have passed the crest of the steep grade. Vehicles behind them are slowed, degrading operations at the least, and contributing to rear-end conflicts and in some cases risky passing maneuvers at the worst.  Truck drivers may also choose to descend grades at slower speeds to maintain better control of their vehicles.  Operations may be degraded for faster-moving vehicles from behind, creating an increased risk of rear-end crashes and risky passing maneuvers.

Another potential safety concern is present when a horizontal curve lies at the bottom of a steep grade (Figure 16).  This combination of alignments increases the risk of severe run-off-road crashes.

Figure 16 is a photo of a downhill view of a road with a horizontal curve in the distance where the road flattens out.

Clarification

The adopted criteria also include achieving a minimum grade.  Grades of at least 0.30 percent are considered necessary to achieve appropriate drainage of the pavement.  Where very mild grades are used for significant lengths of highway, care should be taken to assure the combination of cross slope (see discussion below) and grade are sufficient for good drainage.  A design exception is required when either the maximum grade for a design condition is exceeded, or when the minimum grade cannot be achieved.

Traffic Operations

The combination of grades, including length of grade, and horizontal curvature can have a demonstrable influence on vehicle speeds. One tool for assessing this operational condition is the Design Consistency Module of FHWA’s IHSDM (see Chapter 1). This module produces a speed profile for continuous alignment by direction of travel. It can be used to test alignment variations, and provide a direct operational measure of a design exception for maximum grade.

Substantive Safety

Table 14 illustrates how variations in grade may affect safety on rural two-lane highways. 

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

Summary

Table 15 summarizes the potential adverse impacts to safety and operations of a design exception for grade.

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.

Grade 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 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.
  
  
• Guidelines for Geometric Design of Very Low-Volume Local Roads (ADT ≤ 400), AASHTO, 2001.
  
  
• Highway Drainage Guidelines, AASHTO, 2000.
  
  

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