AI Lines

What is fast_lane.ai?

The fast_lane.ai file tells AI opponents how to drive around your track. It contains:

Racing Line - The optimal path AI cars will follow around the track
Track Boundaries - Left and right edge limits that define where the AI can safely drive
Speed Hints - Information about corners and straights for AI speed calculations

Without a fast_lane.ai file, AI opponents won't be able to race on your track.

Opening the AI Lines Editor

Click the AI Racing Line tool in the toolbar (🏎️ icon) to open the AI Lines panel. You'll need at least one road placed on your track.

Creating an AI Line

Select a road - Choose which road to create the AI line from using the dropdown
Copy Centerline - Click this button to create the racing line from your road's center
Auto-Optimize - Finds the smoothest line that fits between the track edges: wide on entry, tight at the apex, wide on exit, and straight through S-bends where the road allows it

Editing the Racing Line

The racing line appears as green markers connected by a green line. You can drag these markers to adjust the racing path:

Drag a marker - Move the racing line point. Side limits stay put unless Move side nodes with centerline is on
Shift + click - Add or remove that node from the selection
Ctrl + Shift + click - Select every node between the last selected node and this one (closed loops take the shorter arc)
Drag a selected node - Move the whole selected stretch together. Drag Influence blends the unselected nodes at each end of the stretch
Drag Influence slider - Neighboring points that move with a falloff (from the dragged node, or from the selection edge on a multi-select drag). 0 = hard edge

Editing Track Boundaries

Track edges appear as orange (left) and blue (right) markers. These define the safe driving area for AI:

Drag an edge marker - Move that edge boundary point independently
Drag Influence - Also applies to edge markers for smooth adjustments

The edges are completely independent from the racing line - you can adjust them without affecting the racing path.

Panel Controls

Node Spacing - Distance between points along the line (smaller = more detail, larger = smoother)
Road Width Used - How much of the road Auto-Optimize may use (100% = edge to edge, lower keeps the line nearer the middle)
Edge Margin - How close the car's centre may get to the asphalt edge during Auto-Optimize (about half a car width puts the wheels on the edge line)
Drag Influence - Number of neighboring nodes affected when dragging (0-25)
Show Track Edges - Toggle visibility of edge lines and markers
Smooth Line - Apply smoothing to the racing line
Resample - Redistribute points evenly along the line at current spacing

Exporting

Click Export fast_lane.ai to download the file directly, or the AI line will be included automatically when you export your track.

The exported file converts your 3D racing line and edge positions into the distances and data format that Assetto Corsa expects.

Traffic Lanes

The Traffic tab authors a pair of offset AI files for online traffic (AssettoServer), separate from the racing line.

Pick a route with the Route Builder, or use the selected road as the whole route.
Choose Keep right or Keep left, and whether the route is a closed loop.
Click Generate Traffic Lanes. This creates two files: fast_lane.ai (forward) and fast_lane_2.ai (oncoming), each sitting in the centre of one lane (25% / 75% of road width).
Select a file in the list to nudge its markers. Range-select works the same as the racing line: Shift+click to add a node, Ctrl+Shift+click to select the stretch between two nodes. Then Export this file or Export All (.zip).

These downloads do not replace the racing-line fast_lane.ai inside a full track ZIP. Put the traffic files in the track ai/ folder on the server yourself. AssettoServer loads every fast_lane*.ai file; a .aip pack is optional later (rename the zip).

Open routes are allowed. AssettoServer will still close a spline if its first and last points are within 50 metres, so leave more gap if you want a true point-to-point line. Cars that reach an open end without a junction despawn.

Tips for Good AI Lines

Test your line - Drive the racing line yourself to make sure it's smooth and fast
Use Auto-Optimize first - Start with optimization, then fine-tune manually
Smooth corners - Make sure corners flow naturally without sharp kinks
Edge boundaries - Keep edges at realistic track limits - not too tight, not too loose
Higher node spacing - 1.5-2m is usually good; too many nodes can cause jittery AI behavior