NCDOT Roadway Design Unit:

There are several "Methods of Attaining Superelevation" as described in the AASHTO 2004 "Green Book". The below documentation is written for the "Table Method" used in Roadway Designer. The table values are derived from the 2004 "Green Book" pages 167 to 174 for the superelevation rate. The runoff values are derived from page 180 and 181. After obtaining the runoff length base on the design speed and superelevation rate, the values can be adjusted to meet the common practices in Roadway Design such as even increments and rounded superelevation rates. First some aspects of superelevation have to be defined as used in NCDOT Roadway Design.

Superelevation Runoff or Runoff Length - Length of roadway needed to transition from full super to zero percent cross slope high side and vice versa.

Tangent Runout or Runout Length - Length of roadway needed to transition from normal crown to zero percent cross slope high side and vice versa.

Distribution for Simple Curves - For simple curves, one third (1/3) of the runoff length is distributed inside the curve while the remaining two third (2/3) of the runoff length is distributed on the tangent segment. Full super starts 1/3 of runoff length inside the PC or PT point and transitions to the zero percent cross slope high side point on the adjacent tangent segment. Total transition is complete by adding the tangent runout length to the runoff length, going from normal crown to zero percent cross slope high side point and vice versa.

Distribution for Curves with Spirals - For spirals, the runoff length is the same as the length of spiral. Full super is maintained on the total length of curve. Full super starts at the SC or CS point and transitions to the zero percent cross slope high side point at the TS or ST. Total transition is complete by adding the tangent runout length to the runoff length, going from normal crown to zero percent cross slope high side point and vice versa.

Pivot Point or Crown Point - For an undivided facility, the pivot point or crown point is located on the centerline. For a divided facility, the pivot point or crown point is located on the inside edge of travel way (Inside EOT).

e Max - The maximum superelevation rate applicable for the type of facility in consideration. The tables in the "Green Book" is separated out by their e Max value.

e Design - The actual superelevation rate used on a specific curve depending on the e Max chart, design speed, and curve radius.

High Side / Low Side - Depend on the direction of the curve, if the curve is turning to the right (positive deflection), then the high side is the left side and the low side is the right side. If the curve is turning to the left (negative deflection), then the high side is the right side and the low side is the left side.

Click on the below pictures to view a clearer and higher resolution image of the diagrams.

Simple Curve Undivided Facility

Curve with Spirals Divided Facility


		The North Carolina Department of Transportation