Superelevation Calculator
Superelevation banks the roadway cross-section on horizontal curves to counteract outward centrifugal acceleration through gravity and tire lateral friction. Transition runoff (Lr) and tangent runout (Lt) smoothly rotate the pavement from normal crown (-2.0% to -2.5%) through reverse crown to full superelevation (+e_d). This tool estimates preliminary critical transition stations and exports CSVs for verification in Civil 3D.
Calculating critical superelevation transition stations (Normal Crown, Level Crown, Reverse Crown, Full Super) across multi-segment corridors with varying design speeds and e_max limits is prone to manual arithmetic error. Improper runoff rates create drainage flat spots or excessive pavement twist. This tool automates multi-segment calculations and outputs clean Civil 3D tables.
- •ERA Geometric Design Manual 2013 (project applicability to be verified)
- •AASHTO Green Book 2018 (Section 3.3.8: Transition Runoff Design)
- •Highway Geometric Alignment Engineers
- •Civil 3D Corridor & Assembly Modelers
- •Resident Engineers staking out cross-slopes
e + f = V² / (127 · R)e = Superelevation Rate (m/m), f = Side Friction Factor, V = Speed (km/h), R = Radius (m).L_r = [(w · n1 · e_d) / Δ] · b_ww = Lane Width (m), n1 = Rotated Lanes, e_d = design cross slope (m/m), Δ = relative gradient (m/m), b_w = Adjustment Factor.L_t = (|e_nc| / e_d) · L_r|e_nc| = normal crown magnitude (m/m), e_d = design cross slope (m/m).Use these outputs for preliminary checks. They do not constitute official engineering approval. Verify the inputs, method and results against the project criteria before using them for design or construction.
Superelevation Calculator & Civil 3D CSV Generator
Estimate superelevation rates ($e\%$), transition runoff ($L_r$), tangent runout ($L_t$), and transition milestones for horizontal curves across corridor segments. Export preliminary results from entered curves or repair an existing Civil 3D report.
Design Standard Manual Option
Alignment Corridor Segments (Terrain & Speeds)
Define varying terrain sections, design speeds ($V$), and section types along chainage. The engine maps curves to their segment automatically.
Option 1: LandXML Alignment Importer & Departure Audit
Upload LandXML Alignment File (.xml)
Select a Civil 3D LandXML alignment (.xml) to review curve geometry and preliminary screening results.
LandXML screening estimates curve stations. CSV export from this workflow is paused until source stationing can be verified. Use the curve calculator with independently verified geometry to create a preliminary CSV.
Engineering Notice & Review Requirement:
Calculation generated by InfraDigital CAD based on the selected methodology and entered parameters. Project-specific engineering review and verification are required before final design, submission or construction use.
Superelevation Calculator Documentation
Technical specifications, mathematical formulas, and civil guidelines
Which workflow should I use?
Use the curve calculator to screen entered simple circular curves, using the displayed design presets and a centerline pivot. The estimated cross slope and runoff are preliminary and should be compared with the governing design criteria. Use LandXML to review imported alignment geometry. Use CSV repair to recalculate transitions in an existing Civil 3D tabular export and check them against the original geometry.
How are runoff and runout defined?
Tangent runout is the distance from adverse normal crown (NC) to zero outside cross slope (level crown, LC). Runoff begins at LC and ends at full superelevation (FS). Reverse crown (RC) occurs within runoff when the outside lane reaches the magnitude of normal crown and the full design rate exceeds that magnitude.
For an independent design check, use Lr = w × n × bw × ed / Δ and Lt = |enc| × Lr / ed. Use metres for lane width and decimal slopes (m/m) for ed, enc and Δ. See AASHTO Green Book, 7th edition (2018), §3.3.8.2, Eqs. 3-23 and 3-24 and Tables 3-15 and 3-16b. The preliminary curve screen estimates Lr from its presets rather than implementing all these tables.
What does the two-thirds split apply to?
On a simple circular curve, the calculator places two-thirds of Lr on the approach tangent and one-third on the circular curve. At the exit it mirrors that layout around PT. With spirals, the repair workflow uses the spiral endpoints for LC and full slope. Check the resulting rotation rate against the actual spiral length.
Worked check at station 10+200.00
Assume a 3.65 m rotated width, ed = 0.060 m/m, normal crown magnitude 0.025 m/m and a project limit Δ = 0.0045 m/m. The independent calculation gives Lr = 48.67 m. Adopt 50.00 m and calculate Lt = 20.83 m using the adopted length. For an entrance PC of 10+200.00, the resulting stations are NC 10+145.83, LC 10+166.67, RC 10+187.50 and FS 10+216.67. The last 29.17 m rotates the outside lane from +2.5% to +6.0%. The full 50 m runs from LC to FS.
Check station order, both pavement edge profiles near the flat zone and the available distance to adjoining transitions. The curve screen uses a high-side shoulder rollover limit of 7 percentage points. The CSV repair mode follows the source-template shoulder rules. Verify both against the project’s cross-section criteria.
Which CSV button should I use?
Choose Export Preliminary Civil 3D CSV from the entered-curve workflow or Download Repaired CSV from the repair workflow. Compare column order, stations and slope signs with a native export from your Civil 3D release, then verify the result in a copy of the drawing.
Read the full superelevation guide and download the station check sheet.
Developer of InfraDigital CAD
Read the tool-specific source, assumptions and limits before interpreting its output. These browser-based tools are provided as productivity aids for preliminary engineering and drafting checks. Project-specific independent verification and approval by a licensed Professional Engineer (PE / REng) is required before final design submission, drawing stamping, or construction implementation.
Author: Yonatan Abrham
InfraDigital CAD developer
Technical Integrity & Independent Verification: Use the tool for the stated calculation or workflow. Check the applicable edition, input assumptions and limits in the tool documentation.
The user remains solely responsible for verifying that all inputs, soil parameters, traffic projections, and geometry comply with project-specific statutory requirements and geotechnical investigations prior to final engineering approval or construction stakeout.