4.7.3 Superelevation Rate

As a vehicle traverses a horizontal curve, it undergoes a centripetal acceleration that acts toward the center of the curve. Vehicle weight, roadway superelevation, and side friction between the tires and pavement surface sustain this acceleration. The equation that governs vehicle operation on a horizontal curve is:
e + f=V215R
Where:
e =
 superelevation rate, ft/ft
f =
 side friction factor
V =
 vehicle speed, mph
R =
 curve radius, ft
There are practical upper limits to the rate of superelevation.
The Department normally uses a maximum superelevation rate of 6 percent.
 However, a maximum rate of 8 percent may be used where higher superelevation rates or sharper curves are desired. The recommended maximum for facilities where there is a regular occurrence of very-slow moving vehicles, whose operation might be affected by high superelevation rates is 6 percent.
Use of 8 percent should be coordinated with the District Design Engineer prior to implementation and documented in the project files.
To provide designers flexibility in high-speed urban and suburban settings, a maximum superelevation rate of 4 percent may be used instead of a 6 percent or 8 percent superelevation rate.
Freeway facilities are excluded from using a maximum superelevation rate of 4 percent.
4.7.3.1 Methods of Calculating Superelevation Rate
When calculating superelevation on rural and high-speed suburban, urban, and urban core facilities, AASHTO uses a process known as
Method 5
. Method 5 is intended to accommodate overdriving that is likely to occur on flat to intermediate curves. Overdriving on such curves involves little risk that a driver will lose control of the vehicle because superelevation sustains nearly all the lateral acceleration at the average running speed and a large amount of side friction is available for greater speeds. Due to the additional side friction available, Method 5 is used on rural and high-speed suburban, urban, and urban core facilities to provide greater driver comfort and safety.
Where superelevation will be applied to low-speed roads on low-speed rural town, suburban, urban, and urban core facilities, AASHTO uses
Method 2
 superelevation distribution. Method 2 only introduces superelevation after the maximum side friction has been used. Therefore, no superelevation is needed on flatter curves. This method is used on low-speed rural town, suburban, urban, and urban core facilities where, because of various constraints, superelevation frequently cannot be provided. Method 2 ensures driver safety but does not offer the added driver comfort that Method 5 provides.
Table 4-3
provides a summary of scenarios for the use of Method 2 and Method 5. The following assumptions are included in the application of
Table 4-3
.
  • Urban context
     includes Urban Core, Urban, Suburban, and Rural Town.
  • Rural context
     is exclusive of urbanized contexts;
  • Intermediate Speed
     is a range of speeds from 50 – 60 mph that technically falls within what would typically be categorized as a High-speed facility, the Intermediate values are provided in
    Table 4-3
    to give the designer additional flexibility in these transitional areas;
  • The general continuum for the amount of calculated side force on a vehicle in a horizontal curve from greatest to least is as follows:
    • Method 2;
    • Method 5 (4 percent e
      max
      );
    • Method 5 (6 percent e
      max
      ); and
    • Method 5 (8 percent e
      max
      ).
    The designer has the option of selecting a superelevation methodology that either increases the radius and/or decreases the side force for driver comfort on the various facility types listed
    Table 4-3
    ; and
  • Side force should never exceed AASHTO maximums for a given design speed, regardless of the method used.