Information on Alpha angle

As specified in the rotary standards, the Alpha angle is the angle of intersection between the tangents to the rotary's boundaries and the tangents to the boundaries of the entry's roadway. Strictly speaking, you do not define this value but the missing angle that adds up to 200 [grd].

The Alpha angle characterizes the angle of intersection between the direction of the entry roadway and the rotary roadway. In particular, this angle is defined by the position and shape of the central part.
You are advised to create the roads leading to the rotary so that they are centered on the rotary's center or perpendicular to the rotary's roadway. Design the central part so that large Alpha angles of approximately 80 to 90 [grd] are generated. Flat angles lead to higher entry speeds, which might cause more accidents.
If possible, try to avoid Alpha angles that are less than 70 [grd] for safety reasons. If you cannot avoid this, it is essential that the Beta angle is greater than 45 [grd].