What equation is used to find the superelevation runoff length, Lr?

• A. Eq. 3-21
• B. Eq. 3-23
• C. Eq. 3-25
• D. Eq. 3-27

Answer: (B)

The proper equation for calculating the superelevation runoff length, Lr, is based on geometric and hydraulic principles that account for the transition from a normal crown to a superelevated section. Superelevation runoff is critical in designing curves to ensure vehicles can safely navigate from straight sections to curved sections of a roadway.

The specific equation used in design manuals, such as those from the AASHTO Geometric Design of Highways and Streets, typically incorporates factors such as the rate of change of road cross slope, vehicle speed, and the radius of curvature of the roadway. This allows for an accurate assessment of how long a distance is required for the transition, which impacts both vehicle stability and driver comfort.

When determining the correct equation, it's essential to refer to the relevant sections of the design manual, where these formulas are laid out in detail, emphasizing their operational suitability for different scenarios. The calculation for Lr ensures that adequacy in runoff design is aligned with safety and operational efficiency, reflecting the needs for proper drainage and vehicle handling on superelevated turns.