Quick answer: The No Hesi style is a dense AI traffic server on a highway map, usually Shutoko Revival Project. It runs on AssettoServer, not the official Kunos dedicated server, because the official one has no AI traffic feature at all. Your drivers need Content Manager with CSP enabled, and the same cars and track as the server. The good news for anyone hosting one: Assetto Corsa is the lightest game we run, so this costs less than nearly anything else.
What the format actually is
No Hesi is the largest Assetto Corsa server, built around high speed highway driving with dense AI traffic, and it has spawned an entire category of servers that copy the format.
The appeal is specific and worth understanding if you want to run one: it is not racing. There is no grid, no qualifying and no chequered flag. It is driving fast through traffic with other people around, on a map big enough that you can drive for a long time without repeating yourself. That is a different thing from a race server and it attracts different drivers.
Shutoko Revival Project, a recreation of the Tokyo expressway network, is the map the style is most associated with. LA Canyons is the other common one.
Why the official server cannot do this
The Kunos dedicated server runs race weekends. Practice, qualifying, race. It does that well and it has no AI traffic feature.
So a No Hesi style server runs on AssettoServer, a separate community server built for freeroam. That is where the AI traffic, dynamic weather and stronger Steam identity checks come from.
If you have been trying to configure traffic into the official server, that is why it is not working. The feature does not exist there.
What you need on the server
Three pieces, and all three have to line up.
AssettoServer itself, installed in its own directory. Not inside your Assetto Corsa folder, because the two ship different versions of the Steam SDK and they will conflict.
A map with an AI spline. Traffic follows a path defined in fast_lane.ai or fast_lane.aip in the map's ai/ folder. No spline means no traffic, regardless of your configuration.
EnableAi set to true in extra_cfg.yml.
There are prebuilt No Hesi style server file packs available if you would rather not assemble it yourself, which is a reasonable starting point given how much configuration this style involves.
One tuning warning: the default AI settings are tuned for Shutoko Revival Project. Run traffic on a different map and it will likely need its own settings. Odd AI behaviour on a non Shutoko map is expected rather than broken.
What your drivers need
This is where server owners get support messages that are not really server problems.
Joining a server in this style requires Content Manager with CSP enabled. Content Manager is the third party launcher most of the community already uses, and CSP, Custom Shaders Patch, is the client side layer that makes the whole modern freeroam experience work.
For servers using Steam authentication there is a version floor: CSP 0.1.75 or newer, and a recent Content Manager build.
And as always in Assetto Corsa, every driver needs the same cars and track as the server, at the same versions. There is no content download on join.
The practical advice: publish your required content pack and your minimum CSP version wherever you advertise the server. A driver on an old CSP build cannot be fixed from your end, and the failure looks like the server rejecting them rather than a version problem.
The honest bit about the community
The No Hesi format is popular and it is also divisive, which is worth knowing before you build a community around it.
The criticism you will hear is about driving standards: the format rewards speed through traffic, and on a public server that attracts people who ram, block and generally treat other drivers as scenery. That is not a technical problem and no server setting fixes it.
What does help is the thing most successful servers in this style do: a clear ruleset, active admins, and a Discord where people can report behaviour. If you are running one of these for a group of friends it is a non issue. If you are running a public one, plan for moderation from the start rather than after your regulars leave.
The cost, which surprises people
Assetto Corsa is the lightest game we host. The physics that matter run on each driver's machine, and the server is mostly relaying state, so a full server does not need a large machine.
Memory scales with your content list, the modded cars and tracks, rather than with driver count or with the AI traffic itself. A stock content server for a full grid is comfortable on 2GB.
That is why our Assetto Corsa hosting starts at EUR 1.99 a month. A dense traffic server on Shutoko is not an expensive thing to run, whatever the setup effort suggests.
Where you host it matters more here
Racing is the genre where latency is felt most directly. In a survival game 40ms is invisible. Alongside another car at speed, it is not.
We run Assetto Corsa in the Netherlands, North America, Singapore and India. Put the server where most of your drivers are rather than in the geographic middle, because the middle gives everyone mediocre latency instead of most people good latency.
FAQ
What is a No Hesi style server? A dense AI traffic server on a highway map, usually Shutoko Revival Project. High speed driving through traffic rather than structured racing.
Can I run one on the official Assetto Corsa server? No. The Kunos dedicated server has no AI traffic feature. This style runs on AssettoServer.
What do drivers need to join? Content Manager with CSP enabled, CSP 0.1.75 or newer for Steam authenticated servers, and the same cars and track as the server.
Why is there no traffic on my server?
Either the map has no AI spline in its ai/ folder, or EnableAi is not set to true in extra_cfg.yml. Both are required.
Why does the AI drive badly on my map? Default AI settings are tuned for Shutoko Revival Project. Other maps commonly need their own settings.
How much does a traffic server cost to run? Less than most games. Assetto Corsa is light, and memory tracks your modded content list rather than driver count or the AI.
Related reading: setting up AI traffic on AssettoServer, official server vs AssettoServer, or Assetto Corsa hosting from EUR 1.99 a month.