Road cross-slope (side-to-side slope perpendicular to travel) is typically 2% on straight sections. Superelevation is the intentional inward tilt on horizontal curves — typically 4–8% maximum — to provide centripetal force at design speed. Superelevation transitions over a calculated runoff length; verify cross-slope at 25-ft intervals through the transition zone because errors there are most common.
Standard Cross-Slope Requirements
| Road Element | Typical Cross-Slope |
|---|---|
| Straight section (travel lanes) | 2% away from crown |
| Concrete pavement (straight) | 1.5–2% |
| Paved shoulders | 2–6% |
| Curbed sections | 1.5–2% |
Superelevation Transitions
The road transitions gradually from normal crown to full superelevation through two zones:
- Tangent runout: The outside lane rotates from 2% normal crown to 0% (flat). Begins before the point of curvature (PC).
- Superelevation runoff: The outside lane continues rotating from 0% to full superelevation. Typically 2/3 before the PC and 1/3 inside the curve.
Runoff lengths are tabulated in AASHTO’s Green Book or state DOT superelevation tables. Most superelevation errors occur in the runoff zone — verify cross-slope at 25-ft intervals through the transition.
Measuring Cross-Slope in the Field
Total station edge shots: Shoot both edges of the travel lane at each station. Cross-slope = (high edge − low edge) ÷ lane width × 100. Most accurate and best-documented.
Level and rod: Hold rod at one edge, level the rod horizontally across to the other edge, read the difference. Divide by lane width for %. Works well for rough checks.
Digital inclinometer: Place perpendicular to centerline on a smooth surface for a direct cross-slope reading. Best for final pavement surface checks.
Common Errors
- Both lanes sloping the same direction: On a crowned section, each lane slopes away from centerline in opposite directions. Grading both lanes to the same slope creates a severe drainage problem.
- Skipping the runoff zone: Check every 25 ft through the transition — a sharp break at the PC is the most common superelevation error and the hardest to see visually.