Back to Basics Pt. 6: Roadway Design in Civil 3D - Roadway Intersections

In this segment, we will look at some of the tools and methods for modeling intersections between corridors.

- Author Name: Ben Wardell
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Description:
The first intersection we will look at is a T intersection. To make space for the intersection, we will turn off the center regions of the corridor. We also have the option to adjust the station limits in the corridor parameters or we can use the region grips to adjust the limits manually. To provide the space to work in, we will adjust all 3 legs of the intersection.
To begin with, we will select the intersections to drop down, and choose to create an intersection. We choose the location of the intersection, making sure our alignments intersect. In the dialog box, we can provide the Name and how the roadways interact, either with all crowns or with a primary alignment crown. Here we will choose the primary alignment as the east-west alignment. We can either add this corridor to an existing corridor or create a separate intersection corridor.
Next, we can specify the offset parameters. We can provide an input value for the offset or use alignment to control the limits. Here we will specify the width for both sides of the roadways. Then we can specify the turning movement geometry and radius. We then input the assemblies for the segments. Here we can use the default assemblies or specify the assemblies we have already created.
Once the intersection is built, we can inspect for any issues. It looks like the assembly we chose may have some issues, or we need to update the targets. Anytime we change the assembly, the targets are reset. In this case, we can use the assembly we created without the target widths and offsets. Once the intersection is correct and lines up with our proposed design, we can adjust the region limits to close any gaps.
If needed, we can also revise the assemblies that were created with the intersection corridor. Here we want to add a sidewalk to the curb and gutter return. Once we make the changes to the assemblies, we simply rebuild the corridor to view the results.
FOR the next intersection, we have crossing alignments in a 4 legged intersection. We also have a turn lane from the NB lanes to the EB lanes that we want to include in the corridor. We follow the same steps by choosing the intersection command and specifying the intersection location. For 4 legged intersections, we need to specify the primary alignment. Here we will choose the North-South movement. We follow the same steps by providing the offset values or control alignments. In this intersection, we have different values for the curb radius in each quadrant. We can measure the radius from our file as the input. We also want to include a turn lane in our second quadrant. C3D allows us to choose the lane width, radius, the taper length, and the length of the turn lane. We can change the cross slope if needed, but for now, we will keep the default values.
For the assemblies, we will choose the default setting and make modifications later. Once the corridor is complete we can view this in a few ways. We can see the turn lanes that were added. If we needed to update the lane layout we could adjust the control alignments. We can easily add or remove sub-assembly items in the assemblies and rebuild the corridor. We can also make other revisions to the corridor parameters, just as we have with the previous corridors.
In cases where we want to have more control of the intersections, we can use the manual method that utilizes offset alignments or feature lines. To begin with we want to modify the primary corridor regions. At the intersection, we only want to model half the road. We can simply copy the main assembly and remove the lane and shoulder elements in one half. We will update the parameters with the new assembly and re-run the corridor. Now we have the room to model the intersection exactly as we need to.
So we need to create offset alignments that we can use to control the horizontal and vertical location for the curb or edge of the pavement. Here we will choose the alignment to offset and specify the station limits. We can base this off the other corridor regions and measure this in our file. Then we set the side to offset our control alignment. We only want to offset to the right side so we will set the left side to 0. We need to specify the offset distance as well. Here we will set the distance as 32 feet – that is the distance to the edge of the pavement. We want to be sure we choose the profile and we can determine the offset slope. This will be the standard 2% slope.