Design of Roundabouts
using Corridor Models
(Part 1 of 2)
Dynamic vertical modeling
Civil 3D Technical Sales Manager UK and Ireland
Full Geometry Based Design...........................................................4
Step One. – Horizontal Design ........................................................5
Step 2. – Vertical Design, use of a Tilted Plane ..............................6
Step 3. – Using Corridors for Level Information, ICD.....................10
Step 4. – Centreline Vertical Profile Design...................................13
Step 5. – Using Corridors for Level Information, Centrelines.........15
Step 6. – Vertical Profile Design of the Crownlines .......................16
Step 7. – Final Roundabout Corridor Design.................................19
Step 8 – Create a Surface of the Roundabout...............................32
Step 9. – Calculation of Volumes ..................................................35
Step 10. – Dynamic Adjustments ..................................................39
Roundabout design has always been a complex process. And,
because they are
generally expensive to build and take a large amount of space, not
that many designed. Engineers, therefore, are not well practiced in
However, they are becoming more popular. They do have
advantages over traffic signals and maintain free-flowing movement
including Heavy Goods Vehicles (HGVs).
In general, roundabouts can be divided into two main categories;
1. Full geometry-based design
2. Mini roundabouts
This workflow guide will approach both in two separate documents.
This first workflow guide will work up a full geometry-based
roundabout model using corridors.
Please note: The requirements within this workflow are primarily
based on Autodesk Civil 3D 2007 functionality. However it can be
created in 2006 but there are extra benefits from new functionality
Also it is advisable to have a good knowledge of Civil 3D and of the
Throughout the design, the naming of all the civil objects is key to
being able to bring the final design together in a logical manner.
Full Geometry Based Design
This workflow will concentrate on the vertical design for a full
geometry roundabout. It is assumed that the horizontal is relatively
straightforward and can be done using either Civil 3D alignment
layout tools or AutoCAD drafting tools to create the geometry
Methods of design can vary but the main method of vertical design
is the ‘Stockdale Method’ mor also known as 2/3 crown lines.
This method was development by Mr R. Stockdale in the 1970s.
Since then it has become main method of design in the UK.
Its approach is to design an Inscribed Circle Diameter (ICD) and
crown lines that intersect the ICD. Then levels and offsets can be
applied to create the kerb lines, footpaths etc.
From ‘The Journal of the Institution of Highways Engineers, August/September 1978
This approach works especially well with Civil 3D, as the Assembly
technology is able to create all features from a baseline, i.e. the
Also the use of corridor surfaces means that you can graphically
see other intersecting alignments, such as roads etc to enable a
clean accurate tie in.
Step One. – Horizontal Design
Create the ICD, centreline and 2/3 crown lines horizontally using alignments
For detailed guidance on this area of design refer to ‘DMRB TD 1693a’
Below is the example that will be used, refer to drawing
The alignments have been named with a suitable convention to assist in the process
Civil 3D Dynamic Roundabout - Step 1.dwg to start from the beginning or move to Step 2
Step 2. – Vertical Design, use of a Tilted Plane
Refer to drawing Civil 3D Dynamic Roundabout - Step 2.dwg
To start the vertical design, we need to start from the ICD.
So as a guide for levels the use of a tilted plane would give a reference for levels and ensuring drainage. It can also be
used as the actual profile design.
Draw a line through the roundabout as shown below . Using the feature line toolbar, Select the ‘create from object’ and
convert this line to a feature line. It is good practice to create grading features in a separate ‘Site’ from the alignments.
Drape to the ground surface using ‘levels from surface’ and uncheck ‘Insert intermediate gradient break points’
Using the ‘quick level edit’ set the levels and or set a gradient such as 1% down from one side to the other
Create a grading object from the ‘Grading’ menu
Using the grading tools offset the line both sides by -1% to the left and +1% to the right. These values depend on your
requirements. As with the grading object, these values can be amended at any stage.
Automatically created on completion will be a triangulated surface
Resultant surface from the ‘Grading Object’
Create a profile from surface for the alignment ICD and select both the ‘EG’ Surface the grading surface ‘Tilted Plane’
Surface. Assign the ‘Tilted Plane’ Surface profile a different profile style to change colour, in this case, ‘Additional Surface1’.
This creates a dynamic Sine Curve. This acts as either a guide to design the vertical profile of the ‘ICD’ or can be used as
the actual profile in the design. In this example we will use this as the actual profile in the design.
It is not necessary to draw this profile, but it will give a good understanding of what has been created by
the grading ‘Tilted Plane’.
Tilted Plane Surface
Step 3. – Using Corridors for Level Information, ICD
Refer to drawing Civil 3D Dynamic Roundabout - Step 3.dwg (Grading lines are hidden for clarity)
Subassemblies and Assemblies
The subassembly lane type most suitable for this application is ‘Lane with outside Super’. This
subassembly has the ability to widen to an alignment and also raise the edge to a profile. Also if desired you can
set the pavement depths to 0 to omit any pavements you do not require.
Create an ‘Assembly’ for the ICD road
Consists of a lane to each side of the Marker. A kerb and a footpath/verge to the right of the lane
We will use this assembly to create a temporary corridor. This is to create a surface that will be referenced for the actual
design. When creating the corridor, use the ICD alignment, the ‘Tilted Plane’ profile, and the assembly created above.
Name the corridor ‘ICD Temporary Surface’ and set frequency to 5m (see figure below).
From this corridor create a surface from using the ‘Top’ link codes
To clean the workspace in the drawing change the surface style for the ‘ICD Temporary Surface’ to ‘All Off’ and also create
a layer such as ‘Hidden Roads’ and change this corridor to that layer and freeze it off.
In addition set this corridor in the prospector to automatic rebuild. This will rebuild to any changes to the ‘Tilted Plane’
Step 4. – Centreline Vertical Profile Design
Refer to drawing Civil 3D Dynamic Roundabout - Step 4.dwg (Previous Corridor hidden to ‘Hidden Roads’ layer)
Create the profiles for the three centerlines .
Use in addition to the ‘EG’ Surface, the ‘ICD Temporary Surface’.
Then design the new vertical profile for these centerlines and tie into the ‘ICD Surface’.
It is not necessary to create a profile on the kerb face and into the centre of the roundabout as these profiles are only
used in part.
Example profile design.
The profile ties into the carriageway surface of the ICD, then extends the grade of that surface and then can continue to the
Step 5. – Using Corridors for Level Information,
Refer to drawing Civil 3D Dynamic Roundabout - Step 5.dwg
In this section we are going to create a guide for designing the crownline profiles by using corridor surfaces.
These will be used in the profile views to display where 1:40 (2.5%) would be on the Crownline profiles.
Create corridors and corridor Surfaces from the centerlines
Create from an assembly with two lanes (see figure below).
Setting the pavement depths to 0 to omit them, as we do not require quantities from these corridors.
Add these corridors to the ‘Hidden Roads’ layer and turn the surface style to ‘All Off’
Now there is all the level information required to design the crown lines accurately.
Step 6. – Vertical Profile Design of the Crownlines
Refer to drawing Civil 3D Dynamic Roundabout - Step 6.dwg
Now that we have all the reference surfaces we need, we’ll create the crownline profiles. When sampling for the crownline
profiles, make sure to include both the ‘EG’ Surface, add the temporary corridor surfaces that it crosses.
In this example for the ‘North Crownline’, we see the ‘West Temporary Surface’, the ‘ICD Surface’ and the ‘North Surface’.
Use different surface styles to give a different colour for clarity.
To find the intersecting area of the crownlines and the ICD, use alignment labels to add labels that mark out the intersection
of the crown lines to the ICD.
The Horizontal Geometry band also shows this area. Look for the radius of the ICD. In this case 31.35m.
Mark on the profile views, using a vertical line and then offset to the chainage values
When designing the vertical profile ensure that between the two lines, directly follow the surface of the ICD. Then grade out
accordingly. The two other surfaces show where 1:40 (2.5%) would be, so to maintain a regular grade where possible.
Example of vertical profile design
Remaining two crownline profile designs
Step 7. – Final Roundabout Corridor Design
Refer to drawing Civil 3D Dynamic Roundabout - Step 7.dwg
Treat the Roundabout as one corridor model. The corridor will be built from the crownlines with the use of the centrelines
and the ICD for widening lanes and level.
This method will use multiple baselines and regions.
Assemblies required. You should at this point name the carriageway lanes, such as ‘Left’ and ‘Right’. As later in the
exercise you will need to identify the individual subassemblies to target to alignments and profiles.
‘Crown Lines Left Edges’
(Lane on the right will act as a widening, so lane width and slope does not have to be set to a specific value.)
‘Crown Lines Right Edges’ (a mirror of the previous detail)
(Lane at the required width and slope, kerb and footway. It is not necessary for the footway/verge strip to the right as a
COGO point could be added to the resultant corridor surface later in the design)
Create the corridor from a crownline, in this case ‘West to North Centreline’, using a frequency of 5m and set the end
chainage to the intersection of the centerline and the ICD.
Location of intersection of centreline and ICD
Set the targets
Alignments = Subassembly named ‘Right’ to the ‘West Centreline’ (to widen this lane out to the alignment)
Profile = Subassembly named ‘Right’ to the ‘West Centreline’ alignment and profile ‘Vertical’.
Click on ‘OK’ to build the corridor.
Under the corridor properties, add a second region. Choose the same assembly and set the end chainage to the
intersection of the ICD to the second centreline as shown above.
Set the targets to the
Alignment = Subassembly named ‘Right’ to ‘ICD’
Profile = Subassembly named ‘Right’ to ICD alignment and profile ‘Tilted Plane – Surface’
Rebuild the corridor
Add region 3 with the end chainage running out to the end of the alignment.
Set the targets
Alignment = Subassembly named ‘Right’ to ‘North Centreline’
Profile = Subassembly named ‘Right’ to ‘North Centreline’ alignment and profile ‘Vertical’
Rebuild the corridor. This section is complete
Final corridor properties of this section.
Click on the to add a second baseline to add the second Crownline, North to South Crownline and then create this part
of the corridor as above using the same assembly ‘Crown Lines Left Edges’. Targeting region 1 = ‘North Centreline’, region
2 = ICD and region 3 = ‘South Centreline’.
Then repeat for the third crownline ‘West to South Crownline’. Using ‘Crown Lines Left Edges’ assembly. Targeting, region
1 = ‘West Centreline’, Region 2 = ICD, region 3 = ‘South Centreline’
Uncheck the previous baseline while you build each section to save time rebuilding the corridor.
Second baseline of the corridor
Third baseline of the corridor
Finally add the final baseline to create the lane, kerb and footpath/verge area on the inside of the ICD
Create using the ‘Tilted plane’ as the profile
If you are using the ‘Tilted Plane’ surface rather than a design profile set the frequency to a regular interval
and change to ‘no’ on the extra frequency interval positions such as profile geometry etc, as it will create
more intervals than required.
Recheck the remaining baselines of the corridor to ‘On’, set the target surfaces for the earthworks to ‘EG’ and rebuild.
Final Roundabout Corridor
To tidy the arms at the ends, additional baselines maybe required to obtain a clean finish so to run a normal Corridor model
up to the roundabout for each approaching arm.
Step 8 – Create a Surface of the Roundabout
Refer to drawing Civil 3D Dynamic Roundabout - Step 8.dwg
Go to the corridor properties and click on the surface tab.
Create a surface to the ‘Top’ link codes and give a suitable style for the display such as ‘Contours and Slope Arrows’ to
understand the flow of the surface
Add a boundary to restrain the triangulation to the Daylight Feature Line.
(Refer to Workflow – Corridor Surface Interactive Boundaries, www.autodesk.co.uk/civil3D-downloads for further
Set the use type to an outside boundary’
Surface of the Roundabout
3D View of the Surface
Step 9. – Calculation of Volumes
Refer to drawing Civil 3D Dynamic Roundabout - Step 9.dwg
To calculate the cut and fill earthwork volumes, create another surface to the roundabout corridor.
Create the surface to the ‘Datum’ link codes. Also add a boundary to the ‘Daylight Feature Lines’.
This will create a surface to the underside of the paved materials, as this is the true earthworks and the paved materials of
the corridor can be reported separately.
Use draw order to move the top surface to the back so to be able to choose the corridor feature lines for the
Corridor properties, set the style to ‘All Off’ as you do not need to see this surface.
Create a TIN volume surface from the ‘EG’ surface and the datum corridor surface ‘Roundabout Earthworks Datum’
Change the surface style to ‘2D Level Banding’
Click on the surface and properties and the analysis tab
Set to 2 Ranges and click the arrow to calculate the analysis
Set Range 1 from the negative level to 0. Range 2 set level to 0 to the maximum level, see below.
Also change the colours to suit, such as red for cut and green for fill.
This illustrates in green the areas in fill and red for the area in cut.
The image above already shows a reasonable balance of Earthworks.
To check the volume, either use the volume surface tools or click on the properties of the volume surface and statistics tab
Step 10. – Dynamic Adjustments
Refer to drawing Civil 3D Dynamic Roundabout - Final.dwg
The roundabout is fully dynamic in its vertical design.
By amending the ‘Tilted Plane’ grading object will affect the design. The changes to this surface will appear in the related
Additionally if the temporary corridors are set to automatic, these also will update in the profile views.
The only changes to be made are small adjustments to the related profiles.
Above shows the grading object to be raised by 0.5m
The ICD surface has now automatically raised in the centreline profiles
Also the ICD surface has changed on the crownlines
This method is relatively simple to produce a very complicated model using traditional
engineering design methods and gives the designer more time to refine the model as the
whole corridor of the roundabout is truly dynamic.
Traditionally a roundabout would take upto a week to produce and a week to change.
This method brings those changes to within an hour.
Many thanks to the following organisations for their input and support
ACSUG (Autodesk Civil Solutions User Group)
LGCSB (Irish Local Government Computer Service Board)
Many thanks to Wigan Council for their support and providing survey datasets
Wigan Council – Engineering Consultancy