What Is the Highway Curve Minimum Radius & Superelevation Calculator (AASHTO)?
Horizontal highway alignment balances centrifugal overturning forces using banked road slopes and tire-pavement friction.
How Does the Highway Curve Minimum Radius & Superelevation Calculator (AASHTO) Work?
Resolves vehicle weight and centrifugal force vectors along the inclined transverse pavement plane.
Highway Curve Minimum Radius & Superelevation Calculator (AASHTO) Formula & Variables
The core mathematical equation utilized by this calculator is expressed as:
R_{min} = \frac{V^2}{127 (e + f_{max})}, \quad D = \frac{1746.38}{R}, \quad a_{lat} = \frac{v^2}{R}
Minimum curve radius R_min equals design speed squared over 127 times sum of superelevation fraction e and maximum side friction factor f_max.
How to Use the Highway Curve Minimum Radius & Superelevation Calculator (AASHTO)
- Enter roadway design speed in km/h.
- Specify maximum allowable superelevation percentage (e).
- Optionally supply side friction factor.
Step-by-Step Example Calculation
100 km/h Highway with 6% Superelevation
Input Values:
designSpeedKmh:100
superelevationPercent:6.0
sideFrictionFactor:0.12
Worked Steps: With 6% banking and f_max = 0.12, the minimum safe horizontal radius is 437.4 meters (1,435.2 ft) with 1.76 m/s² lateral acceleration.
Understanding Your Result
Minimum Radius (R_min): Sharpest permissible curve radius in meters/feet.
Degree of Curvature: Arc definition of sharpness per 100m arc.
Lateral Acceleration: Transverse G-force experienced by occupants.
Factors That Affect the Result
- Design speed scales required radius with the square (V²); steeper banking allows tighter turns.
When Should You Use This Calculator?
- Transportation civil engineering, interchange ramp layout, racetrack banking design, and railway curve calculations.
Assumptions & Limitations
- Assumes passenger vehicle center of gravity and uniform pavement condition throughout the curve transition.
Frequently Asked Questions
Calculation Accuracy & Reference Note
Standard AASHTO Green Book metric horizontal curve formulation.