Blackburn's formula does this by sketching the arc of your car's turning capability into a full circle, then using the center of the circle to create the right-angle triangles Pythagoras loved.

Next, the center of the pie is erased by drawing a white circle, and another arc is drawn to provide a visual border between the interior circle and the peripheral pie slices.

A radian is the Angle subtended by an arc of a circle whose length is just equal to the radius.