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1. Sometimes in road design, user may face an error prompt on ‘invalid arc angle’ as per image below;

  • You can download the project sample here


2. This error occur may due to several possibilities where the data cannot be compute mathematically.

3. Therefore, they are few suggestions to overcome this issue;

a. Change the spiral length to 0 m

b. Reduce the design speed

c. Reduce the design radius

4. Please go to Horizontal Alignment >> Curve >> Spread Input


5. As per image attached above, you can see that, you can change the spiral length, design speed and design radius.

a. Change the Spiral Length


  • As you can see that spiral length for IP 1 is changed to 0 m for both in and out. Now you can proceed to road width

b. Reduce the design radius


  • As image above, you can see that the Design Radius is changed to 90 and now you can proceed to road width
  • 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.

c. Reduce the design speed


  • The above image shows that you may change the design speed to allow for the arc angle to be  valid and proceed to road width

6. Furthermore, the design speed, design radius and spiral length is interrelated with each other to calculate the arc angle.

7. You may refer table below for guideline;