Sometimes in road design, user may face an error prompt on ‘arc angle for IP is not a valid value’ as per image below;
You can download the project sample here
This error may be due to several possibilities where the data cannot be computed mathematically. If you view the value of ArcAngle at the RHS panel under Properties tab, the ArcAngle is actually a negative value. Hence, this is why you have the “Calculated arc angle for IP is not a valid value” error message and there are several approaches available to resolve this error.
Reduce The Spiral Length #
Initially, the spiral length for IP 1 is 40.500 m for both in and out, now the ArcAngle is a negative value. The ArcAngle value is shown at the RHS panel on the Properties tab.
As you can see that spiral length for IP 1 is changed to 0 m for both in and out, now the ArcAngle has been changed to a positive value.
Adjust The Design Radius #
Initially, the design radius for IP 1 is 30 m, now the ArcAngle value shown at the RHS panel on the Properties tab is a negative value.
As image below, you can see that the Design Radius is changed to 120 m and now the ArcAngle has been changed to a positive value. Any increase in the radius of the curve beyond this minimum radius will allow you to reduce the side-friction factor, the superelevation rate, or both.
You may refer table below for guideline;