Chapter 4 - Elements of Design

4.5. Superelevation

4.5.1. Maximum Superelevation Rates

The maximum superelevation rates (emax) shall be limited to those values depicted in Table 4.9. Maximum Superelevation Rates. Superelevation rates higher than those in Table 4.9., but within AASHTO Green Book guidelines shall require a design variance.

Horizontal alignments are composed of tangent sections connected by arcs of circular curves (GDOT does not normally use spiral curves).

Table 4.9. Maximum Superelevation Rates
Setting	Maximum Superelevation Rates (emax)*
Urban (Curb and Gutter) Roads (DS < 45mph)	4%
Suburban / Developing Areas	6%
Rural (Non Curb and Gutter)
Paved Roads	6%
Unpaved Roads	Reverse Crown
Interstates, Expressways, L/A Facilities
Rural	8%
Urban	6%
System-to-System Ramps
Rural	8%
Urban	6%
Exit-Entrance Ramps	8%
Free Flowing Loop Ramps	10%
Long Ramps with STOP	8%
*The maximum allowed values (emax) for usage on Georgia roadways, as designated by GDOT. In general, GDOT does not require superelevation on low-speed urban roadways or roadways with a design speed of 25 mph or less

Vehicles traveling in a circular path counter the centrifugal force that would cause them to leave the road through a combination of two factors:  lateral friction between the vehicle’s tires and the road, and superelevation. The maximum rates of superelevation used on highways are controlled by four factors: 

• Climatic conditions (i.e., frequency and amount of snow and ice) 
• Terrain conditions (i.e., flat, rolling, or mountainous)
• Type of area (i.e., rural or urban)
• Frequency of very slow-moving vehicles whose operation might be affected by high superelevation rates

Superelevation requirements for maximum superelevation rates (0.04 to 0.12-ft./ft) for various design speeds (15 mph  to 80 mph) are provided in the AASHTO Green Book (2004), Chapter 3. Elements of Design – Superelevation Tables. GDOT has designated the values in Table 4.9. as the maximum values (emax) for use on Georgia roadways.

It is important for designers to realize that the minimum curve radii and maximum superelevation rates depicted in the AASHTO Green Book  are extremes and should be avoided wherever possible.

The emax values presented in Table 4.9. requires the use of the more moderate design value ranges for curvature and superelevation. In certain situations, such as those described below, the emax values in Table 4.9. may require further reduction:

• Wherever practical, consideration should be given to maximizing curve radii and minimizing superelevation rates on curves which include bridges. This is due to the increased potential for icing. Where constraints do not exist, an emax of 4% should be utilized.

• Wherever possible, the maximum superelevation rates on roadways within an intersection should be limited to 4% (2% for urban areas with crosswalks). Wherever possible and when applicable in intersections, superelevation cross slopes of one roadway should be coordinated with the mainline profile grade of the intersecting roadway.

• Where traffic congestion or extensive development acts to restrict top speeds on a rural roadway, a maximum rate of superelevation of 6% should be used.