To ensure Allsite tools can reliably build and maintain roading networks (Road Corridors, intersections, knuckles, cul-de-sacs, sharp bends), corridors must be created using a consistent Civil 3D workflow. This section describes the supported approach and the minimum setup required before running the Civil 3D Intersection Wizard.

Note - you can run Level AI without road corridors (using Allsite's Layout Assist), for this follow instructions here.

Run the 'Validate Corridor' tool to validate the DWG and road corridors within it meet Allsite requirements for Level AI / Service AI.

Main Workflow

I. Supported methods and current limitations

1. Road Corridors

Corridors can be created using standard Civil 3D alignments and assemblies.

Custom combined sub-assemblies are not currently supported by Allsite.ai (support may be added in the future).

Allsite.ai does not currently support custom combined sub-assemblies. Support may be added in future.

2. Alignments

Road centerline alignments are drawn as normal.
Do not extend alignments past the end of a cul-de-sac or T-intersection. Overrun geometry can cause issues—trim alignments to the road region end.

II. Prerequisites (before intersections)

Allsite requires that each road centerline alignment has:

An Existing Ground profile, and
A Design profile (preliminary design is fine; commonly a manual straight-line design profile), and
A corridor created with regions, and
The corridor’s vertical baselines explicitly set to use the design profiles.

Only after this is complete should you run the Civil 3D Intersection Wizard.

III. Step-by-step required workflow

1) Create road centerline alignments

Create your road centerline alignments as usual.
Confirm alignments are trimmed correctly at dead-ends/intersections (no overshoot).

2) Create profiles for every alignment (Existing + Design)

For each centerline alignment, create:

Existing Ground (EG) profile
Design profile (preliminary is acceptable; manual straight-line is supported)

You can do this manually, or use the Allsite tool that loads profile views and creates both profiles (EG + Design).

3) Create corridors with regions

Create a corridor for the alignments using standard assemblies.
Ensure the corridor is built with regions (do not leave the corridor as a single undefined region).

4) Set vertical baselines to use the Design profiles (required)

Use the Allsite tool for setting vertical baselines.
Confirm each corridor baseline is using the intended design profile as its vertical baseline.

Critical: If design profiles are not set as the vertical baselines, curb return profiles may lose or fail to maintain links to the offset alignments’ profiles when vertical baselines change.

5) Only then: run the Civil 3D Intersection Wizard

With corridors built and vertical baselines correctly assigned, you can now create intersections reliably.

Note: Best practice recommendation (even if using connected alignments/profiles)

Civil 3D intersections created with connected alignments + connected profiles generally maintain more robust relationships.
However, Allsite recommends still creating and assigning preliminary design profiles first, then setting them as vertical baselines, before running the intersection workflow. This reduces profile-link instability and keeps corridor behavior predictable as edits occur.

IV. Validate your drawing before Level AI / Service AI

After corridor setup (and anytime you make major edits):

Run Allsite → “Validate Corridor”
This checks that the DWG and its road corridors meet Allsite requirements for Level AI / Service AI.

Road Network Features

I. Intersections

An Intersection represents the area where two or more roads meet and connect.
Intersections can be created using the Civil 3D Intersection Wizard or manually, provided the same geometric and profile principles are followed.

Supported creation methods

Intersection Wizard (recommended): uses offset alignments and dynamic curb return profiles.
Manual: allowed if the same geometry + profile linking rules are respected.

Prerequisites

Corridors exist for the intersecting roads, built with regions.
Each centerline alignment has an Existing and Design profile.
Corridor vertical baselines are set to use the Design profiles.
If not, curb return profiles may not maintain links to offset alignment profiles when vertical baselines change.
Side road rules must be disabled.

Requirements

Offset alignments and offset curb returns must be used (Feature Lines are not supported for defining curb returns).
Curb return subassemblies: use the “Gutter Edge” insertion point when creating intersections manually (Civil 3D default).
Lane subassembly required on curb returns

Feature Lines are not supported to define curb returns.

Curb returns must have a 'lane' subassembly, this extends mesh lines towards the target alignment. In the future support will be added for this type of curb return / cul-de-sac.

Corridors that do not use intersections / connected alignments and do not have curb returns are not currently supported (ie where feature lines are used to manually set levels around intersection). Support will be added 2026.

Profiles

Having created the corridor and intersections with the design profiles as vertical baselines will allow LevelAI to update the design profile automatically without needing to update the profiles used by intersections.

Allsite does not support secondary road profile rules. Before running Level AI results for the first time, the rules must be disabled for every intersection and any ruled locked pvi, unlocked. This is because Level AI is automatically grading the roads at these tie-ins between roads and the extra points added can mess with the design.

Cul-de-Sacs

A Cul-de-Sac represents a terminal road bulb or turning area at the end of a road.
It typically consists of a single alignment defining the bulb geometry and a main road alignment (centerline) that terminates at the bulb’s center.

Definition and Requirements

To be supported by the plugin, cul-de-sacs must comply with the following requirements:

Location: Found at the end of a road or a closed terminal section.
Alignment Definition:
- The Cul-de-Sac should be represented by one centerline alignment defining the tangential bulb arc or circular end.
- The Main Road Alignment (used as target) must end/start at the center of the cul-de-sac.

Alignment Continuity: The Cul-de-Sac alignment should connect and align or overlap tangentially with its adjacent region, depending on the curb return subassembly used (insertion point: Curb Back or Gutter Edge).
Region: A Cul-de-Sac will have one baseline and region
Region Association:
- The Cul-de-Sac Alignment should start and end at the same station of the Main Road Alignment
- The same centerline alignment is shared between the adjacent region (as baseline) and the cul-de-sac region (as Target).
Target:
- The target must be set to the Main Road Alignment and be curved to the center.
Other supported geometry: Sided Cul-de-Sac

Note: Using the Road Main alignment as cul-de-sac baseline is not supported.

Knuckles / Bulbs

A Knuckle (or bulb) represents a sharp corner or tight curve in a road network, typically used to model the turning point between two connected alignments.
Knuckles can appear with or without a bulb (flared widening), but their geometric and targeting setup follow the same general rules.

Definition and Requirements

To be supported by the plugin, knuckles must comply with the following requirements:

Location: Positioned on a sharp curve of a road.
Curve Definition: The curved road must be defined by two independent connecting alignments (To be used as Targets).
Knuckle Definition: The knuckle itself must be represented by two alignments forming its outer and inner edges.
Alignment Continuity: Both knuckle alignments should connect and align tangentially with their adjacent regions, depending on the curb return subassembly used (insertion point: Curb Back or Gutter Edge).
Regions: A Knuckle will have a baseline and region per its respective alignment(Inner/Outer).
Targeting: Targets must be set to the centre of the road alignments. (See example image showing region and targets.)
Bulb/No-Bulb Cases: Both bulb and non-bulb cases are supported; the construction principle remains the same.
Curved alignments through knuckles are also supported.

Note: using road main alignments as baselines for knuckles is not supported yet.

Detailed Drawing Example

Before creating the knuckle, have your (separate) alignments intersecting. Decide on your curb type now, as it will affect the flow of operations. Allsite.ai currently supports Gutter Edge, or Back of Curb curbs.

To make them tangential, draw Polylines straight out from your chosen corridor line.

Repeat for the same side of the other corridor region. Don't draw too far as we want to place a curve.

Flatten your new polylines to remove any z values, then use the fillet tool to connect them with a curve.

Convert this curve to an alignment, add a surface profile, then repeat for the other side.

Next, create two basic assemblies labelled 'Left knuckle' and 'Right knuckle'.

In Tool Palettes, add a UrbanCurbGutterGeneral. Change your curb side (Left knuckle assembly means right side curb), and choose the insertion point based on how you constructed the corridor at the start.

In Tool Palettes, add in LaneSuperelevationAOR.

Repeat this assembly creation for the other side.

Add your previous two alignments as Baselines in Corridor Properties, randomly assigning your L / R assemblies.

In Corridor Properties, change the Target of both of these alignments to just the intersecting middle road alignments.

After rebuilding the corridor, if you notice a side looks odd, change the assembly for that alignment. Make sure to re-assign a target again, then rebuild the corridor one last time.

Click for detailed steps for creating a knuckle: