Superelevation transitions: runoff, runout and station checks

Define the pavement edge and signs first

In this example, slopes are measured from the centerline out toward each edge. Negative means that the edge is lower than the centerline. At normal crown (NC), both edges are below the centerline. At level crown (LC), the outside edge is level with the centerline and the inside lane still drains toward its own edge. At reverse crown (RC), both lanes lie in one plane. With equal widths and a normal crown magnitude of 2.5%, that means +2.5% on the outside and −2.5% on the inside. At full superelevation (FS) the slopes are +6% and −6% in this example.

Tangent runout is the distance from NC to LC. Runoff is the distance from LC to FS. RC is a milestone inside runoff when the selected full cross slope exceeds the normal crown magnitude. If the two magnitudes coincide, RC coincides with FS. If the full rate is lower, the outside lane never reaches a distinct RC milestone. Check which edge is outside by looking in the direction of increasing station: on a right curve it is the left edge. A convention based on a single left-to-right axis uses different signs, so map signs before comparing files.

Calculate lengths with one unit convention

The relative gradient measures how quickly the pavement edge rises or falls relative to the rotation axis along the alignment. With a fixed width and a uniform rate of change in cross slope, the edge height is width times cross slope, and the relative gradient is the change in that height divided by the station interval. Use metres for width and decimal slopes in m/m for both the selected design rate and the relative gradient. For example, 6% is 0.060 m/m and 0.45% is 0.0045 m/m.

Lr = w × n × bw × ed / Δ, where w is lane width in metres, n is the number of lanes rotated about the controlling reference line, bw is the applicable lane adjustment factor, ed is the full design cross slope and Δ is the limiting relative gradient. For one lane on each side of a centerline pivot, the controlling width in the example is one lane and the adjustment factor is 1.0. If the same rotation rate is used through runout, Lt = (|enc| / ed) × Lr, where |enc| is the magnitude of the adverse normal crown in m/m.

Choose the rate and gradient from the governing criteria before doing the arithmetic. The balance equation e + f = V² / (127R), with V in km/h and R in metres, is a demand check. By itself it does not select the project cross slope because the agency determines how to distribute demand between banking and side friction. In the 2018 AASHTO Green Book, §3.3.5 and the applicable design table, such as Table 3-9 for a 6% maximum, address rate selection. Section 3.3.8.2.1, Eq. 3-23 and Tables 3-15 and 3-16b address runoff and lane adjustment. Table 3-15 in this edition is the lane adjustment table. Section 3.3.8.2.2 and Eq. 3-24 address tangent runout. Use the metric entries and check any agency supplement. [1, 2]

The online tool also includes a preliminary curve screen that estimates cross slope and runoff from its presets. It does not reproduce every Green Book design table. Compare its lengths with your independent calculation rather than assuming identical output from the same speed. For Ethiopian projects, check the project edition of the ERA Geometric Design Manual and the adopted cross section and road class. A Green Book example does not establish ERA criteria.

Worked example: station the entrance and exit

Assume a right-hand curve with PC at 10+200.00 and PT at 10+380.00, one 3.65 m lane on each side, a normal crown of 2.5%, a full cross slope of 6%, and an illustrative limiting relative gradient of 0.0045 m/m. The selected inputs illustrate the method and are not a speed-specific table entry. The minimum runoff for those inputs is 3.65 × 0.060 / 0.0045 = 48.67 m. Adopt 50.00 m, then recompute the runout using that adopted length so the outside edge rotates at the same rate through both segments. This gives Lt = 0.025 / 0.060 × 50.00 = 20.8333 m. Keep the extra digits until the station schedule is displayed.

Use the site’s two-thirds tangent and one-third curve allocation for this example. Apply the split to runoff only. The entrance LC is 33.33 m before PC and FS is 16.67 m after PC. Because the outside lane rotates linearly from flat to +6%, RC is 20.83 m after LC. NC is 20.83 m before LC. FS is therefore 29.17 m after RC, and the change from +2.5% to +6% uses the remaining 3.5 percentage points of the runoff. The exit reverses that sequence around PT. The selected allocation is an example convention. Use the governing agency’s placement criteria for the actual alignment. [1, 3]

The exit column reads upward in station order. At PC and PT the outside lane is at approximately +4.0%. Check the adopted relative gradient from edge heights: the outside edge rises 0.219 m between LC and FS, so the gradient is 0.219 / 50 = 0.00438 m/m, below the example limit. At RC the outside edge is 0.09125 m above the centerline. At NC it is 0.09125 m below. Subtract the station values at the displayed precision to check ordering, but calculate differences from unrounded values if the last centimetre matters.

Download the worked station check sheet and compare its outside edge elevations with the Road Cross-Section Coordinates tool. The CSV is a hand-check sheet with explicit signs and units. Autodesk’s tabular editor expects its own column layout.

Check drainage, adjoining curves and shoulders

Inspect a short length on each side of LC. Near that station transverse drainage weakens on the outside lane. On a centerline grade of +0.40%, the example entrance high-side edge grade is about +0.838% while it rotates upward at +0.438% relative to the pivot. At the exit, with that same centerline grade, the edge grade becomes approximately −0.038% during the opposite rotation. Thus a profile that climbs continuously can still produce an edge grade reversal. Check the grades just outside the transition and at changes in pivot, lane width, curb and gutter details. Confirm the direction of flow and where water can leave the paved surface. A single profile grade check at LC can miss a local low point along the gutter.

At full superelevation a +6% outside lane next to a −4% shoulder has a 10 percentage-point slope break when both values use the outward convention. The calculator’s simple curve screen illustrates a 7 percentage-point limit. Under that assumption the shoulder would need to be at least −1% at that station to keep the break at or below seven points. The station screen does not prove that the revised shoulder drains or satisfies the project rule. The CSV repair mode reproduces a separate source-template shoulder rule. Check shoulder cross slopes and rollover independently before issuing either file.

For closely spaced curves, compare the normal-crown recovery station for the first curve with the start of runout for the next. If they overlap, draw the signed slope profile for both edges together and check the agency’s treatment for compound or reverse curves. Separately check whether the entrance and exit runoff of a single short curve leaves any full-slope plateau. For a spiral layout, use the project’s TS, SC, CS and ST geometry and check that the resulting edge gradients fit the adopted lengths. Extending or shortening runoff in a table without rechecking edge elevations can conceal a change in relative gradient. [1, 3]

Hand off to Civil 3D and review the result

Start with a copy of the alignment drawing. Export the existing tabular data from the Civil 3D release you will use and compare its station representation, region identifiers and lane and shoulder columns with the file you intend to import. Use Export Preliminary Civil 3D CSV in the calculator when working from entered curves. Use Download Repaired CSV only when working from a source export in the repair workflow. A match in column names alone does not establish that the same physical edge is represented. Read one right curve and one left curve in the tabular editor and inspect the rotation in section views at normal crown, level crown and full slope. [4]

Before issuing the drawing, compare at least one imported station from each transition with the independent schedule, verify station equations and check for duplicate or out-of-order rows. Then rebuild the corridor and compare finished edge elevations with the design profile and drainage paths. Save the adopted cross slopes, gradient, pivot, placement rule and source references with the check sheet. This makes the calculation reproducible when the curve radius, profile or typical section changes.

References

1. AASHTO. A Policy on Geometric Design of Highways and Streets, 7th ed., 2018, chapter 3, §§3.3.5 and 3.3.8.2, Eqs. 3-23 and 3-24, Tables 3-9, 3-15 and 3-16b. ISBN 978-1-56051-676-7.
2. FHWA Office of Federal Lands Highway. Superelevation Tables, design spreadsheets based on the 2018 Green Book.
3. Massachusetts Department of Transportation. Project Development and Design Guide, chapter 4, Horizontal and Vertical Alignment, §4.2.4 on runoff, tangent runout and placement.
4. Autodesk. Civil 3D 2026 Help, To Import Superelevation Data, column layout and import procedure.