Enter the design speed, superelevation, and side friction factor into the calculator to determine the transition radius for a road curve. The transition radius is the radius of the curve that is needed to transition from a straight path to a circular path or vice versa.

Transition Radius Formula

The following formula is used to calculate the transition radius:

R = V^2 / (15 * (e + f))

Variables:

  • R is the transition radius (feet)
  • V is the design speed (mph)
  • e is the superelevation (decimal)
  • f is the side friction factor (decimal)

To calculate the transition radius, square the design speed, then divide by the product of 15 and the sum of the superelevation and the side friction factor.

What is a Transition Radius?

A transition radius is the radius of the curve that is used to provide a gradual transition from a straight section of a road to a curved section. This is important for vehicle stability and comfort, as it allows the vehicle to enter and exit the curve smoothly without abrupt changes in direction. The transition radius is designed based on the speed of the road, the superelevation (the banking of the road), and the side friction factor, which accounts for the friction between the tires and the road surface.

How to Calculate Transition Radius?

The following steps outline how to calculate the Transition Radius:

  1. First, determine the design speed (V) in miles per hour (mph).
  2. Next, determine the superelevation (e) as a decimal.
  3. Next, determine the side friction factor (f) as a decimal.
  4. Next, gather the formula from above = R = V^2 / (15 * (e + f)).
  5. Finally, calculate the Transition Radius (R) in feet.
  6. After inserting the variables and calculating the result, check your answer with the calculator above.

Example Problem:

Use the following variables as an example problem to test your knowledge.

Design speed (V) = 60 mph

Superelevation (e) = 0.08

Side friction factor (f) = 0.15