Alaska DOT Superelevation Example

TBC Work Processes (Info Only) TBC Corridor Modeling Work Process
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In a training last week, I was asked to explain how the Alaska DOT Superelevation Details for a highway project should be applied. In the examples provided the DOT specs show the following details

  • The Superelevation diagram shows that at the start and end of the superelevation transitions, that they apply vertical curves to the superelevation diagram. The Vertical Curve length equals the speed of the road in Feet. i.e. for a road speed of 55mph the length of the vertical curves is 55’.
  • It is uncommon in AK to encounter road design that includes spirals in the alignment. The alignments are all Arc and Line elements. The alignments are typically defined with PIs with inscribed curves, so the alignment data entry method will be the PI based method.
  • Because there are no spiral elements in the alignment, the advanced method of entering the superelevations is used in the TBC alignment editor.
  • The superelevation diagram shows that the DOT provide the Runout Length and the Runoff Length and the Runoff % Outside the curve (in the Tangent section approaching the PC at the start of the curve or the Tangent Section departing from the PT at the end of the curve. i.e. where they show Runoff as 75% in the Plan and Profile drawings that value is the % of the Runoff in the Tangent section, leaving 25% in the curve section of the alignment.
  • In some cases the curves are close to each other in the alignment (in the same direction), i.e. you have a right hand curve - straight - right hand curve combination. Where the curves are too close to each other, they show that the runout of the first curve PT Tangent = 0 and the same for the second curve Runout in the PC Tangent as 0. In this case, the Left side of the road (outside of a right hand curve) superelevates from +6% down to 0% on the Left side of the road and from -6% to -2% on the Right Side of the road, and then immediately starts to increase from 0% to +6% on the Left and from -2% to -6% on the right side.

In the videos below we show how to read and interpret the plans and how to enter the data for the normal superelevations and the superelevations where the Runout length equals 0.