Modeling Roundabout Intersections

The release of AutoCAD® Civil 3D® 2010 included a very desirable feature—a toolset for roundabout design. Although everything was included for creating two-dimensional geometry— including alignments, pavement marking, and signage—corridor modeling of the roundabouts seemed to be missing. Join me, for a little discovery of how to utilize the Intersection Design wizard for developing intelligent, 3D roundabout corridor models (though the roundabout shown below might be a little aspirational.


Getting Started

First things first, you must start out with alignments. The suggested route to minimize the number of times you must add additional approaches is to utilize four separate alignments at the point of intersection. Use station control to modify labels if you must, or just take extra time later adding additional approaches.


For the purposes of this article, we’ll work with a simple 4-way intersection. Whether or not your specific application of the roundabout tools requires a more complex configuration, the point is to explore the transition of it to a Civil 3D corridor.

Creating the Roundabout Geometry


The roundabout tools are located on the Home tab of the ribbon, on the Create Design panel, under the Intersection command drop-down. Select the Create Roundabout option. From there, you will be prompted to select the center of the roundabout which should be the same as your alignment intersection.

You’ll then be prompted to select the approach(es). Pick each alignment (this is where using four separate alignments simplifies the process, as you’ll be limited to two approaches otherwise). Select the Enter key when done and you’ll be brought into the create roundabout wizard.


There are several pages within the wizard that will step you through each component of its construction. The first page allows you to select an .xml file with predefined design parameters for standard roundabout radii. This .xml file can be customized to your heart’s content to match national or regional codes. This initial page is where you may also modify drive widths, inner and outer pavement markings, and alignment styles/names. Select Next once you have completed these fields.

The second page of the wizard involves setting the dimensional values and styles for entering and exiting approach lanes. You need to be sure these values do not exceed the overall radius of the roundabout; otherwise you will receive an error and incomplete design once completing the wizard. Use the predefined radius values as a guide for what should work under these circumstances. There are separate settings that can be defined for each approach alignment, or you can simplify the process, should your design permit it, by applying the settings across all approaches. Select Next once you’re done.

Continuing on with the wizard (don’t worry, though this dialog may seem intimidating, every aspect of it is broken down into individual components) define the settings for approach islands. The same rules apply to this page of the roundabout wizard as the approach lane parameters—use predefined roundabout radius information to help in defining the exact geometry you want to apply.

It’s worth noting when going through this process, you can save the changes you make in the fields back into the .xml file you have been working with or create an entirely new one, customized to your specific design needs. You can also go back into the wizard to edit settings after the roundabout has been completed.


The last roundabout wizard page is dedicated to the additional pavement marking and signage you want automatically applied to your design. The blocks used for the signage are multi-view and will display properly when viewed in plan, section, and perspective views for rendering purposes as well. You can substitute your own blocks to be used as well to meet local codes and change their placement within the roundabout area.

Although you’ve likely already noticed this on previous pages of the wizard, you can define minute details—even down to the paint striping widths used. Select the Finish button when done.


You should now have a finished, two-dimensional roundabout complete with pavement marking and signage (if you chose to include them) in your drawing. The image above should give you a pretty good idea of what this looks like. Feel free to select any portion of the roundabout and select the Edit Roundabout option from the Civil 3D contextual ribbon at any time to make changes that may be necessary along the course of your project or if you ran into any problems with the resulting output. This may also include adding more approaches (if you had only two alignments instead of splitting them at the intersection) or a slip lane for uninterrupted travel for drivers making a “traditional” right-hand turn (or left turn for designs in the UK, Australia, India, parts of Africa, et al). These options are also available within the roundabout contextual ribbon or the intersection command drop-down on the Create Design panel of the Home ribbon tab.

Building a Corridor (Not So Fast…)

We now want to utilize the alignments and other geometry to design the corridor. However, don’t get too far ahead of yourself—remember that the roundabout wizard created only two-dimensional geometry. Before jumping into the intersection design wizard, you’ll first need to create design profiles for the approach lane alignments, as well as the roundabout centerlin


Building a Corridor (For Real This Time)

With our horizontal and vertical geometry all defined and in place, it’s time to jump right into the intersection design wizard. It, too, is also located on the intersection command drop-down. After picking the intersection tool, you’ll be prompted to select the intersection point in the drawing. It sounds pretty simple, but cluttered labels and overlapping linework can make this frustrating. Use layer isolation, zooming, and object snapping to avoid some of the hassle associated with missing the exact point of intersection of the roundabout and approach alignments. Just remember, Civil 3D is doing exactly what you tell it to do, and where.


You’ll need to go through this process for each approach in your roundabout design. I’ll explain the process once, assuming the same or similar methods will be used for each additional approach intersection.

Civil 3D Pro Tip: Be descriptive in the naming of your entities. These designs can start getting very complex in a short amount of time and the last thing you want to do is go on a goose chase trying to figure out what “Alignment – (1) (Copy) and Alignment – (2) Intersection (4)” actually represents in your drawing. Good styles and settings can assist in avoiding a scenario such as this, but you should be diligent in ensuring elements are easy to understand for someone who hasn’t seen this project before.

With that in mind, on the first page of the intersection design wizard, naming each intersection you go through by the roundabout alignment name + the approach. Alignment name should offer more than enough information to pinpoint its location and purpose. Make certain you maintain the crown for the primary road as it would make for a pretty bumpy drive on the roundabout otherwise. With those, and the label/style settings defined, select the Next button.

In the next page of the intersection wizard, you’ll be prompted with several options for changing which alignment is the primary, setting offset parameters for each lane in the intersection, curb return horizontal parameters, lane slope parameters, and curb return profile parameters. Let me break that down just a little bit for you.

Here you can set exact widths for left and right lanes for both the roundabout and approach alignments. You can also optionally define the lane widths on offset alignments (which you already have from the roundabout creation wizard). I sometimes like using predefined offset alignments, but there are two things to be cautious of when doing so. First, the corridor generated is perpendicular to the baseline alignment and linework such as curbs and gutters don’t generate smoothly through curved or variable offset widths. Second, for you to actually generate a corridor from these offset alignments you must have profiles defined for them already. If either of these criteria puts a cramp in your process, use a set width for the lanes and go back later for targeting in your corridor options dialog.

In the curb return parameters dialog, you can set up the radius, chamfer, or three-point arc dimensions to be utilized on your roundabout at each quadrant of the intersection. Although you already set offset alignments or lane widths (with targeting later), the curb return parameters are important for setting a vertical profile for corridor regions where targeting must be applied from the edge of pavement back to the approach centerline and edge of roundabout travel way for an accurate and smooth surface. Set the radii just as your roundabout wizard had generated them and it should make for some easy cleanup work later with corridor grip editing and redefining targets.


The lane slope parameters dialog is another important aspect in how your roundabout intersection will be constructed. If you utilized offset alignments for your lane widths, you must use the corresponding profiles within this dialog box in order to build the intersection corridor. However, if you opted for offset widths to be refined later, simple grade % values are all that are needed here. Keep in mind your roundabout centerline likely requires a positive slope on the inside lane and a negative slope on the outside lane and it should be defined as such here.

The last dialog to be concerned with on the second page of the intersection design wizard is the curb return profile parameters. This dialog determines how far, if at all, your corridor region is extended beyond the curb return radii for the quadrants of your intersection. It is advisable that whichever end of the curb return radius is on the roundabout portion of the intersection, which could be the incoming or outgoing end depending on which quadrant is current, should have the option to extend the curb return profile as “no.” The end of the curb return radius which is part of the approach alignment could and likely should be extended along the entire length of the taper for the approach.

Apply these settings for the curb return profiles to all quadrants of the intersection. Also note if your approach alignment extended beyond the roundabout centerline, you have been working with a four-way intersection and only two of the quadrants actually apply. The others will be an easy fix later by simply deleting the baselines and regions from the corridor properties dialog box after the intersection has already been built.

When done with the curb, return to the profile dialog box, select OK, and click the Next button on the intersection design wizard to proceed to the final page.
With this third and final page of the intersection design wizard you can generate a new corridor or add a region to an existing corridor, name the region, select assemblies that should be applied to each region of the intersection corridor, and/or import assembly group settings used in previous designs.


If you didn’t already have an assembly set to import or available in your current drawing file, they can easily be modified later to update the corridor or swapped out in the corridor options dialog for entirely new, more applicable assemblies. Click the create intersection button once you are done adjusting these settings to complete the first approach intersection.

Repeat this same process for any additional approaches you have within your roundabout design. Similar settings, dimensional values, and assembly sets make the process go faster each time.

The Final Product


Now that you have your approach intersection built from the roundabout alignments and target geometry, you should end up with a corridor resembling the image below.

It’s also always good practice to add a surface to your corridor and check for any irregularities or design problems with the Civil 3D object viewer. Just select the surface and/or corridor entities in your drawing space and select the object viewer option from the contextual ribbon or from the right-click shortcut menu. The corridor and surface should look similar to the image below. Targeting, profile, and assembly adjustments may be necessary to develop an accurate model.


Bryan Tanner (Indianapolis, IN) is a Civil Designer and CAD Manager at First Group Engineering, Inc. focusing on training, implementing, and designing with AutoCAD Civil 3D, Map 3D, and the entire Autodesk Infrastructure Design Suite. He is the blog manager for the AUGI AutoCAD Civil 3D Technical Advisory Committee, manages two local user group chapters, AUGIndy and C3DIG, and runs You can find him discussing Autodesk products and the Civil Engineering industry on Twitter using the handle @Indydrafter.

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