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Traffic designations determine what path a dwarf will take, not what their final destination is. If a dwarf's task takes them somewhere, only a hard barrier will prevent them from going there. Since movement in narrow hallways is slowed by dwarves moving past each other, traffic may become an issue once a settlement begins to grow in size.
Some traffic problems can be prevented by good fortress design, such as making hallway which are likely to be high use double or more width and placing stockpiles close to relevant workshops. Other difficulties may be reduced by designating traffic areas.
Vegetation (saplings) will tend to die if repeatedly trampled upon, leaving dead saplings or shrubs and quickly exposing bare soil. This may be desirable as trees block dwarves' and caravans' paths, but unchecked traffic may trample entire areas of forest regrowth.
Only your dwarves will obey your traffic designations, and then only when they can. If a job requires that they go into a restricted zone, they will. Other creatures will not recognize your traffic designations, and even domestic animals or caravans will follow their own pathing.
Setting Traffic Areas
The do combination sets traffic areas, which are zones used to manipulate the movements of dwarves. Traffic areas can be designated as high, normal, low, or restricted. When walking from one point to another, dwarves consider these designations in finding the shortest path. High level traffic costs the pathfinding AI 1 point per square, Normal (default, undesignated) costs 2 points per square, Low costs 5, and Restricted costs 25. You can change the default values in init.txt, or per-fortress values with do.
- It is often a good idea to set any water source in a biome with seasonal freezing to a Restricted area so your dwarves will be less likely to be caught on it when it melts.
- Some dwarves get disturbed if they walk through a butcher's shop and see an animal being slaughtered, so you may wish to designate the shop as Restricted.
- If an area occasionally gets flooded, or is dangerous for some reason, routing dwarves around it could be lifesaving.
- Setting High traffic areas along roads outdoors prevents vegetation from being needlessly trampled.
- An important use of traffic designations is to restrict movement in the tile where a ballista's firing arrow originates. This will prevent tragic siege training accidents. Note that pets can and will be killed by firing ballista even if movement is restricted.
Setting Restricted does not forbid a dwarf from traveling over those squares, but rather makes them willing to walk around them – for the normal cost table, 12.5 times further, or up to 25 times longer if there is an alternative high-traffic path. If you have an area that absolutely must not be stepped on by dwarves, consider walls.
Traffic designations only affect path preferences when pathfinding. Dwarves generally choose their jobs without weighing the pathfinding costs. For example, one cannot use traffic designations to direct a dwarf to confine his digging to a specific area. He will still take whatever path necessary to get to the job he has chosen to work.
Additionally, traffic designations cannot be used to restrict where a dwarf will stand when building/digging. In other words, traffic designations will not prevent a dwarf from placing himself on the outside of the fort when the last tile of a moat or wall is completed. In some cases, a workaround is to place a wall where you want the dwarf to avoid standing, and suspend it. This prevents him from standing on that tile while building/digging. When the job is finished, the suspended wall may be canceled.
Using Traffic Areas to Improve Framerate
In cavernous rooms that handle large amounts of through traffic but have a small number of exits (a large central dining room, for example) designating a few high-traffic paths ("freeways") between the doors can help reduce the pathfinding cost for dwarves who are just passing through. There may also be benefits to adding low-traffic edges to these freeways to keep the search algorithm from looking for shortcuts. Likewise, any large dead-end room that branches off a major hallway should have the area around its doorway marked low or restricted traffic to prevent dwarves from searching it for shortcuts. As noted above in Limitations, this should not affect the dwarves who have a legitimate reason to hang out in the dining hall or visit the storage room - they will path to their destination regardless. Users may see up to a 10% increase in FPS by implementing these changes throughout their fortress.
Due to what seems to be a minor bug rather than a feature, it's not only possible but quite easy to create one-way ramps - and with those, one-way halls, stairs, and any other restricted path you can create from one controlled access point to another.