Quick answer: AI traffic needs two things. A fast_lane.ai file (or fast_lane.aip) in the map's ai/ folder, and EnableAi set to true in extra_cfg.yml. On the client side, players joining a server with Steam authentication need Custom Shaders Patch 0.1.75 or higher and a recent Content Manager. And before any of that: do not extract AssettoServer into your Assetto Corsa folder, because the two ship different versions of the Steam SDK and they will conflict.
Why AssettoServer exists
The official Kunos dedicated server runs race weekends: practice, qualifying, race. It does that well and it has no AI traffic feature at all.
AssettoServer is a separate community server built for freeroam. It adds AI traffic, dynamic weather through CSP, stronger Steam based identity checks, and it fixes a set of security issues in the original server. If you want a cruise server on Shutoko or LA Canyons with traffic around your drivers, this is the software that does it.
They are not two versions of the same thing. Picking the wrong one is the most common reason a server does not do what its owner expected.
The installation mistake to avoid first
Do not extract AssettoServer into your Assetto Corsa installation folder.
Assetto Corsa and AssettoServer use different versions of the Steam SDK, and putting them in the same place makes those conflict. Keep them in separate directories. This is documented and still catches people, because putting server files next to game files feels like the natural thing to do.
Getting AI traffic running
Two steps, and both have to be right.
1. The AI spline. Place fast_lane.ai, or fast_lane.aip, in the map's ai/ folder. This file describes the path traffic follows. Without it there is nothing for the AI to drive along, and you get an empty road no matter what your config says.
2. Enable it. Set EnableAi to true in extra_cfg.yml.
You can load more than one spline if the map supports it, which is how servers get traffic on multiple routes rather than a single loop.
A tuning note worth knowing up front: the default AI settings are tuned for Shutoko Revival Project. If you run traffic on a different map and it behaves oddly, that is expected rather than broken. Other maps commonly need their own settings, so budget some time for adjusting rather than assuming defaults will transfer.
The client side floor
This is where server owners get support tickets that are not really server problems.
For servers using Steam authentication, joining players need:
- Custom Shaders Patch 0.1.75 or newer
- Content Manager, a recent build
CSP is the client side mod layer that most modern freeroam and traffic setups depend on, and it is also what makes dynamic weather work, including smooth transitions and rain.
The practical consequence: if a driver on an old CSP build cannot join, that is not something you can fix from the server. Publish your minimum CSP version alongside your server details so people update before they try, rather than after they fail.
Dynamic weather comes with CSP
Worth knowing because it is a genuine reason to run this setup rather than the official server.
Together with Custom Shaders Patch, AssettoServer supports dynamic weather with smooth transitions and rain. On a cruise server that runs for hours, weather that changes over the session does more for atmosphere than any single mod.
It is not free though. Rain and weather transitions are more client side work, so drivers on marginal machines will feel it before your server does.
How much server does this need
Assetto Corsa remains light compared with survival or shooter games, and traffic does not change that dramatically. The server is relaying state rather than running the physics that matter, which happen on each driver's machine.
Where memory does climb is content: large modded car and track collections, not driver count and not the AI itself.
For most freeroam servers, that means a small plan is genuinely enough, which is why our Assetto Corsa hosting starts at EUR 1.99 a month. It is the lightest game we host and pricing it like a survival game would be dishonest.
Latency matters more here than in most games
Racing is where latency is felt most directly. A 40ms difference that nobody notices in a survival game is very noticeable when two cars are side by side into a braking zone.
We run Assetto Corsa in the Netherlands, North America, Singapore and India. Put the server where most of your drivers are. If your community is genuinely split across continents, two servers beats one in the middle, because the middle gives everybody mediocre latency instead of giving most people good latency.
FAQ
How do I add AI traffic to an AssettoServer server?
Put fast_lane.ai or fast_lane.aip in the map's ai/ folder and set EnableAi to true in extra_cfg.yml. You can load more than one spline where the map supports it.
Why is my AI traffic behaving strangely on a non Shutoko map? The default AI settings are tuned for Shutoko Revival Project. Other maps commonly need their own settings, so expect to adjust rather than assume the defaults transfer.
What CSP version do players need? 0.1.75 or higher, plus a recent Content Manager, for servers using Steam authentication. Publish your minimum version so drivers update before they try to join.
Can I install AssettoServer in my Assetto Corsa folder? No. They use different versions of the Steam SDK and will conflict. Keep them in separate directories.
Does AssettoServer replace the official dedicated server? It is separate server software, not a mod for the official one. Use the official server for structured race weekends and AssettoServer for freeroam and traffic.
How much RAM does a traffic server need? Less than most games. Memory tracks your modded car and track collection rather than driver count or the AI itself.
Related reading: official dedicated server vs AssettoServer, or Assetto Corsa hosting from EUR 1.99 a month.