Superelevation and runoff on the PE Civil Transportation exam

Superelevation problems ask how far to tilt a curve, how fast it can be driven, and where on the plan the tilt starts and finishes. None of it is in the Handbook, whose Horizontal Design section is geometry only. It all comes from Chapter 3 of the Green Book (AASHTO's A Policy on Geometric Design, 7th ed., 2018), supplied for both the current and the April 2027 exam windows.

What you actually need to own

• The curve equation. 0.01e + f = V²/(15R), with V in mph, R in ft and e in percent. Work it three ways: the minimum radius (e_max with that speed's maximum f), the side friction a given curve demands, or the highest design speed a curve supports. For the last one, check each candidate speed against its own maximum f, which drops as speed rises.
• e_max is not e_d. e_max is the agency's cap, set by climate, terrain, area type and slow vehicles. Where snow and ice are factors, the Green Book calls about 8% a logical maximum: a slow vehicle on ice slides down the slope, toward the inside. e_max only picks the table. Read e_d at the tabulated radius equal to or just smaller than yours, which rounds e up; no interpolation is needed. Low-speed urban streets use Method 2 and their own table.
• Runoff length. L_r = (w n₁ e_d/Δ) b_w. Here Δ is the maximum relative gradient, set by the design speed. n₁ is the number of lanes rotated: an undivided road rotated about its centerline turns half its lanes, so a two-lane road rotates one. b_w adjusts for rotating more than one lane.
• Tangent runout. L_t = (e_NC/e_d) L_r. It removes the adverse crown at the same edge rate.
• Placement. On a tangent-to-curve transition, the Green Book reports agencies placing 0.5 to 0.8 of the runoff on the tangent (most use about two-thirds), so the problem states the share. With a spiral, the whole runoff happens over the spiral and the runout sits just ahead of the TS. Between reverse curves that return to normal crown, the tangent carries each curve's tangent share of runoff plus both runouts.
• The cross-section sequence. Rotating about the centerline, the section goes from normal crown, to outside lane level (where the runout ends and the runoff starts), to reverse crown (one plane at the normal slope), to full superelevation. Until the plane section, the inside lane keeps its normal cross slope.
• Axis with a median. Rotate the whole width as one plane only for medians of 15 ft or less. Holding the median level and rotating each roadway about its median edge works at any width and fits best from 15 to 60 ft. Treat the roadways separately for medians of 60 ft or more.

Where people lose points

• Runoff at e_max instead of e_d. In one problem, the e_max misread gives 192 ft of runoff where the design rate gives 120 ft.
• Relative gradient from the posted speed. The posted limit plays no part.
• Miscounting lanes rotated. A six-lane undivided section rotated about its centerline turns three lanes, not two.
• Leaving e out of the speed check, or entering 5% as 5.0 instead of 0.050.
• The wrong e_max table, or the adjacent row.
• Treating a TS like a PC. With a spiral, none of the runoff goes on the tangent, and the runout is not as long as the spiral.
• Placing the share backward. At the PT end, full superelevation ends one curve-share of the runoff before the PT, not at it or past it.
• Flat grades through the transition. On a curbed street the drainage criteria can push the flattest acceptable profile grade well above 0.5%.

How to study it

Take one curve: radius, design speed and e_max. Read e_d, get L_r from the equation and from the table, and confirm they agree. Compute L_t, place the runoff at a stated share, and station all four cross-section states. Then compute the edge-to-edge difference at a station between two of them. Redo it for a six-lane undivided road and a divided road with a 40-ft median. The PC and PT come from horizontal curve stationing; the drainage grade ties to vertical curves.

Part of the Horizontal Design area of the PE Civil Transportation exam. → Start practicing free — the whole practice bank, free with an account.