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DifficultybeginnerAdded2027.0Statusdraft

Intersection Design (Civil 3D essentials)

Design Intersection Automatically​

Standard Civil 3D contains an intersection wizard, unfortunately it has certain limitations. Devotech developed a new Intersection Wizard which is part of Devotech iDAS. For more details see the T-Intersection Wizard chapter.

Design Intersection Manually​

The image below shows a T-intersection layout with road descriptions. Notice that the Side Road centreline stops at the Main Road centreline, not at the edge of the Main Road:

The workflow below assumes that the road corridors and road top surfaces have been created, see the Create Top Surface from Corridor chapter for the detailed steps to create corridor top surfaces.

  1. The Side Road must be tied vertically to the Main Road. Create a profile from the main road top surface by going to Profiles – Create Profile from Surface in the menu bar:

Or use the Home – Profile – Create Surface Profile command in the ribbon:

  1. Select the Side Road alignment and the Main Road Top surface and click Add:

  2. Change the profile style to Top Surface so it will be visible in the side road profile view and click OK:

  3. Zoom to the Side Road profile view and switch the line weight on to highlight the Main Road Top surface profile:

  4. Tie the Side Road vertical alignment to the Main Road top surface (use Edit Profile Tools toolbar):

  5. To design bellmouth horizontal alignments you can either draw the polylines and convert them to alignments or you can create alignments by layout. The workflow below describes the second option. Go to the Alignments – Create Alignment by Layout command:

OR in the Home tab select the Alignment – Alignment Creation Tools command:

  1. Specify an alignment Name (for left bellmouth BML, for right bellmouth BMR). As an alignment Type use Curb Return (Civil 3D term for a bellmouth). All the bellmouth alignments will be listed under Curb Return Alignments in the Prospector. The centreline type will also work for bellmouth alignments. As an Alignment Style use No Plot Style | Curb Return which is automatically in the C3D-No Plot Objects layer and this style also shows the alignment direction. There is no need to display chainage labels so for the Alignment label set use _No Labels:

  2. For the bellmouth alignment tangents use Fixed Line (Two points):

  3. Draw the tangents on top of the Side Road and Main Road edge of travelled ways (ETW). For the left bellmouth alignment draw the alignment from the Side Road to the Main Road. Tangents do not need to be connected, they will be connected with the bellmouth curve:

  4. For the bellmouth alignment curve use Free Curve Fillet (Between two entities, radius):

  5. Follow the command line prompts. When selecting the First Entity, select the first tangent in the chainage direction. You can click on a direction arrow to select the tangent:

  6. For the Next Entity select the second tangent:

  7. For the curve solution angle use Lessthan180:

  8. Specify the radius:

The created horizontal alignment:

  1. Create an alignment for the right bellmouth by using the same steps as described for the left bellmouth alignment, the alignment direction should be from the Main Road to the Side Road. This only allows the use of one assembly for the bellmouths on both sides. If the right bellmouth alignment was designed from the Side Road to the Main Road, then two bellmouth assemblies would be needed.

The right bellmouth alignment:

Alignment direction arrow detail:

Created alignments in the Prospector:

  1. The bellmouth vertical alignment must be tied to the Main Road and Side Road top surfaces. Create profiles from the corridor top surfaces along the bellmouth horizontal alignment with the Create Profile from Surface command in the Profiles menu:

OR use the Profile – Create Surface Profile command in the Home tab of the ribbon:

  1. For the Alignment select BML, select the Main Road and the Side Road top surfaces (multiple surfaces can be selected with the SHIFT or CTRL keys) and click on Add:

  2. Change the Style to Top Surface and click on the command Draw in profile view:

  3. In the General tab, as a Profile view style, use Curb Return:

  4. Use the default settings for other tabs and in the Data Bands tab, under Select band set use _No Bands and click on the Create Profile View command:

The created profile view (yellow vertical lines indicate the location of the horizontal alignment curve start and end points):

  1. Do the same for the bellmouth aligment on the right hand side (BMR):

  2. Create the bellmouth vertical alignment with the Create Profile by Layout command in the Profiles menu:

OR use the Profile Creation Tools command in the Home tab of the ribbon:

  1. Select the left bellmouth profile view:

  2. Use default settings and click OK:

  3. To design the bellmouth vertical alignment, it must be tied to the roads' top surfaces. The section between the yellow vertical lines is the section at the horizontal curve, thus the vertical alignment should not tie to top surfaces (you can use a straight tangent in this section). For small intersections (typically in township development projects) use tangents without curves. For large intersections, tangents with curves can be used. Multiple PVIs can be used to tie the vertical alignment to the road top surfaces:

  4. For the left bellmouth vertical alignment, notice that multiple PVIs were used between the beginning of the profile view and the first vertical yellow line (the side road is in the vertical parabola at the chainage close to the intersection therefore more PVIs were needed to tie the vertical alignment to the road top surface):

  5. Design a vertical alignment for the right bellmouth:

  6. Shorten the Side Road corridor to the furthest bellmouth curve (you can grab the Side Road corridor grip point and snap to the curve start/end):

  7. Snap to the furthest curve start/end point:

Shortened Side Road corridor:

Notice that the bellmouth profiles from the Side Road top surface were adjusted automatically:

  1. Extract an ETW (Edge of Travelled Way) feature line from the Main Road corridor by going to the Corridor – Utilities menu and selecting the Create Grading Feature Line from Corridor command:

  2. Select the Main Road ETW on the same side as the location of the intersection:

  3. If you want to use the ribbon, select the Main Road corridor ETW (Edge of Travelled Way) on the same side as the location of the intersection in the model space and use the Feature Lines from Corridor command in the contextual ribbon:

  4. Use the settings as per image below (it is better not using a Site to avoid interactivity between feature lines):

The created feature line:

  1. Create an assembly for the main road at the intersection. This assembly should not have a kerb and cut/fill subassembly on the right-hand side. Copy the Main Road assembly with the standard AutoCAD Copy command, only select the assembly centreline (vertical line), do not select the entire assembly and subassemblies with a selection window:

  2. Change the name to Road with Int on Right and delete the kerb and cut/fill subassembly on the right-hand side:

  3. Go to the Main Road Corridor Properties, Parameters tab, right-click on the corridor region and use the Split Region command from the menu:

  4. Specify the split points in model space. These points are the locations where the bellmouth alignments connect to the Main Road edge:

  5. Once you confirm the selection by pressing Enter, the Corridor Properties window will appear again on the screen. Change the assembly for the middle region to Road with Int on Right and close the Corridor Properties window:

The Main Road corridor with the applied intersection region:

  1. Create an assembly for the bellmouth regions. Make a copy of the Main Road assembly, and change the Name to BM, delete the kerb and cut/fill subassembly on the right side and the layerworks on the left side:

  2. Change the layerworks subassembly codes for the inside and outside top points, because the bellmouth assembly runs along the bellmouth alignment the inside point code must have an ETW code and the outside point must have a Crown code:

  3. Go to the Side Road Corridor Properties, Parameters tab and click on the Add Baseline command:

  4. Set the BML alignment as the Horizontal alignment:

  5. In the Profile column click on the <Click here…> button:

  6. Specify the bellmouth vertical alignment:

  7. Right-click on the created baseline and use the Add Region command:

  8. For the Assembly use the BM assembly:

  9. Click on a plus sign next to the baseline name to display the new region:

  10. Set the start station by clicking on the chainage value in the Start Station column:

  11. Select the point where the bellmouth curve starts:

  12. Set the End Station by clicking on the chainage value in the End Station column:

  13. Select the point where the bellmouth curve ends. It is advisable to choose a point a few millimetres back, because if the main road is in the horizontal curve a geometry discrepancy could cause the bellmouth target to be missed (targets will be explained further).

  14. The end station selected few millimetres back:

  15. Set the frequencies by clicking on the ellipsis button in the Frequency column:

  16. Set the frequencies along tangents and curve increment to 2m. If the corridor is not smooth along the bellmouth curves (curves with a small radius), decrease the frequency to 1m. Click on the white plus sign to add an extra frequency point. This point is at the location where the Side Road centreline crosses the Main Road ETW:

  17. Set the targets by clicking on the ellipsis button in the Target column:

  18. Set the Target Surface:

  19. Click on <None> in the Width Alignment row to specify horizontal targets for the layerworks subassembly:

  20. You must specify two horizontal targets. For Select object type to target use Alignment, select the Side Road alignment and click on the Add button:

  21. Change Select object type to target to Feature lines, survey figures and polylines and click on the Select from drawing command:

  22. Select the feature line extracted from the Main Road ETW:

  23. If you are prompted to name the feature line, select the first option:

  24. Two objects must appear in the Selected entities to target area:

  25. Click OK to return to the Target Mapping window and click on <None> in the Outside Elevation Profile row to specify the vertical targets for the layerworks subassembly:

  26. Set Select object type to target to Profiles, as an alignment use the Side Road alignment, as a profile use Side Road vertical alignment and click on the Add button:

  27. Change Select object type to target to Feature lines, survey figures and polylines and click on the Select from drawing command:

  28. Select the feature line extracted from the Main Road ETW:

  29. Two objects must appear in the Selected entities to target area:

  30. Close the Set Slope or Elevation Target window and the Target Mapping window by clicking OK to return to the Corridor Properties window and click on the Add Baseline command to add the right bellmouth baseline:

  31. As an alignment select the BMR alignment:

  32. In the Profile column click on the <Click here…> button:

  33. Specify the bellmouth vertical alignment:

  34. Right-click on the created baseline and use the Add Region command:

  35. As an Assembly use the BM assembly:

  36. Click on the plus sign next to the baseline name to display the new region:

  37. Set the start station by clicking on the chainage value in the Start Station column:

  38. Select the point where the bellmouth curve starts, keep in mind the alignment runs in the opposite direction. It is advisable to choose a point a few millimetres back, if the main road is in the horizontal curve a geometry discrepancy could cause the bellmouth target to be missed.

  39. Start station selected few millimetres further along the alignment:

  40. Set the end station by clicking on the chainage value in the End Station column:

  41. Select the point where the bellmouth curve ends.

  42. Set the same frequencies and targets as for the left bellmouth region and rebuild the corridor.

Final intersection:

  1. To switch the bellmouth alignments and ETW feature line off, freeze the C3D-No Plot Objects layer:

Result:

  1. To check if the intersection was created correctly, display the contours for the Main Road and Side Roads' top surfaces. The contours must match:

Adjust Intersection Vertically​

If the Main Road vertical alignment changes, users must adjust the Side Road and bellmouths' vertical alignments. This applies to intersections created automatically and manually. Sometimes it might be faster to recreate an intersection automatically than to adjust the vertical alignments.

The side road vertical alignment can be adjusted directly, but to adjust the bellmouth vertical alignment you must switch off the intersection regions in the Side Road corridor properties otherwise it will be difficult to tie the bellmouth vertical alignment to the Side Road's top surface. The image below shows a bellmouth profile view after the Main Road and Side Road vertical alignment adjustments. The top surface follows the bellmouth vertical alignment because the bellmouth is part of the Side Road corridor:

  1. Go to the Side Road's Corridor Properties, Parameters tab and switch off the intersection baselines:

  2. The bellmouth profile view after the intersection corridor regions were switched off:

Adjusted bellmouth vertical alignment:

  1. Switch on the intersection baselines in the Side Road Corridor Properties and rebuild the corridor to display the adjusted intersection.