Traffic is the flow of vehicles along roads.
Cities: Skylines individually tracks the passage of every vehicle through your city's road system. These include service vehicles, citizens' private transportation, and freight. It is important to manage the flow of traffic on your roads, as a blocked road causes delays in services reaching certain areas, and an increase in noise pollution.
Roads with heavy and/or congested traffic can be observed visually, simply by looking at the flow of vehicles on the roads. Long tailbacks will be fairly obvious. Watching the flow (or lack thereof) of traffic at a junction or intersection can provide clues as to the cause, and solution, of a given traffic flow problem.
Additionally, Cities: Skylines has a traffic info view, which can be inspected by using the Traffic button on the info view menu. Clicking it overlays a gradient of green to red on the roads, depending on the density of the traffic in the area. Heavy traffic (red areas) are not necessarily problem areas in terms of traffic flow; a well designed road system will have the capacity to deal with its zones of high demand. A "Traffic Flow" rating for the whole city is also displayed in percent. Higher numbers indicate better flow. A second info view can be accessed with the Traffic Routes button. Clicking on a road or path with this selected in the "Route" tab will show all of the traffic currently planning to pass through there. The "Junctions" tab allows toggling traffic lights and stop signs. The final tab, "Adjust Roads", allows you to adjust the road, primarily for the road name. A line will appear to highlight the road, with dots on either end that can be dragged to adjust the road. There will also be a dot where you clicked on the road somewhere in between the ends. You cannot drag an end point past that dot. Click elsewhere on the road to move it, then continue moving the end dot.
Vehicles trying to drive from one location to another will select their route, including the choice of lanes, at the outset of the journey. This route is always the fastest legal route. This calculation assumes that the vehicles will be able to travel at the speed limit of every road segment, i.e. it takes no account of other traffic. This is usually, but not always, the shortest route - vehicles may choose a slightly longer route if it travels down higher speed limit roads. Vehicles will not change their planned path, or even their planned lanes, if they encounter traffic. Vehicles will only change their journey plan if their route is made impossible by changes made to the road network while they are en route.
Because of this behaviour, a new route for traffic to bypass an overloaded intersection will only function as intended if, irrespective of traffic, it represents a quicker route.
Since entire routes are pre-planned, a newly constructed road may not see immediate use.
Avoiding traffic problems
There are two basic ways of alleviating traffic in a city, reducing the number of vehicles and increasing capacity.
- Public transportation means that journeys which would have been done by car or motorbike are made by other methods. If these are bus journeys, then multiple citizen's private vehicles are replaced by a much smaller number of buses. Public transport by train, metro, monorail, ferry, blimp or cable car takes the journey off the road system altogether. To encourage citizens to use transit you can: use the Free Public Transportation policy to allow citizens to travel on transit with no expense (although they pay for it indirectly through taxes). The "Prefer Ferries" policy can increase ferry usage as well. Another option is to add express lines to current transit lines. The express lines only stop at major stops en route. However, when this comes to trains, an express train may be held up by a local train boarding and exiting passengers at another station. To fix this problem, you can add tracks that go around the stations (loop sidings) that express trains don't use. Finally, a player can remove parking spots from roads. Most buildings have small parking lots, but that is rarely enough to accommodate all vehicles using it. Citizens park on the side of the road if other spots are occupied. However, a player can line a road with grass, trees, and even bike lanes to remove parking spots. This will cause the nearest parking spot to be farther away, which may cause transit to be faster for the citizen. The end result will be more citizens using transit.
- An overloaded public transport system can cause problems of itself, with citizens waiting many in-game days to catch overcrowded trains, cable cars, ferries, metro vehicles, trams, monorail vehicles, and buses. Also, too many buses or trams in a small area can actually cause congestion. This can be fixed by using custom public transport vehicles from the Steam Workshop that have higher capacities than the default vehicles.
- The cargo train terminal and cargo harbour effectively provide public transport-like solutions for cargo's transport through your city and can be effective at moving/removing the burden of freight traffic from your roads. The Cargo Hub is a combination of both of these buildings and allows freight to transfer between ship and train without involving trucks on the road, as well as having access to roads for truck traffic. The Cargo Airport and Cargo Airport Hub provide the same services, except utilizing aircraft instead of ships. While these services may reduce truck traffic on the road in some areas, they also can produce large quantities of cargo truck traffic in other areas, particularly immediately adjacent to cargo stations.
- Pedestrian paths and bike paths are another way to make citizens stop using the roads by providing them with walking-based alternatives. They are placeable, and citizens will use them to avoid crossing streets, which also lessens traffic.
- Rezoning, the nature of the traffic burden of residential, commercial, industrial and office zones are each slightly different and redistributing these throughout your city may help reduce a traffic problem. Inevitably, light zones place less of a toll on your roads than heavy ones do. Green Cities adds specializations that can reduce cargo traffic on the roads. The office specialization "IT Cluster" generates some goods that are somehow distributed without using cargo trucks on the road. The commercial specialization "Organic and Local Produce" creates buildings that require fewer cargo deliveries.
- Fewer/smarter intersections: Intersections between roads are necessary, but inevitably slow down the flow of traffic - vehicles are required to slow down and stop for traffic travelling in other directions, and for traffic lights (which are automatically generated at most intersections). A nightmare scenario can occur if the queue at one intersection extends backwards far enough to block traffic at another intersection, causing a cascading effect.
- Although fewer intersections may make some journeys more circuitous, with fewer intersections vehicles are more likely to be able to travel down major roads at speed, reaching their destinations quicker. Increasing the spacing between intersections reduces the likelihood of the nightmare gridlock scenario described above.
- Roundabouts are often (but not always) a good solution to traffic flow problems at an intersection of multiple roads.
- Lane management: Upgrading roads to provide more lanes allows a greater volume of traffic, increases the speed limit of the roads, and helps separate turning and non-turning traffic at intersections.
- This however is not a "silver bullet" for all congestion problems, predominantly due to the following effect: Vehicles will always get into turning lanes at some distance before the intersection at which they wish to turn, and will not alter this behaviour in light of the traffic situation. If junctions are too close together these turning zones can overlap, causing a large tailback in the turning lane, which often contrasts starkly to the clear lanes beside them. This is another reason for spacing out intersections.
- Traffic distribution: When choosing routes, citizens will choose the quickest route from A to B. For this calculation they assume there is no other traffic on the road. Being stubborn creatures, they will not alter their route (or even their lane choices) once they've started, regardless of how much traffic they encounter.
- The challenge for a mayor is how to manipulate this behaviour to produce a working road network. This may involve creating new, alternative routes for some traffic - for example providing a direct highway connection for an industrial zone's imports and exports to prevent freight driving halfway across your city to an existing connection. Alternatively deleting roads may be useful (see also the case for removing intersections above), forcing traffic to take journeys which are longer, but travel by higher capacity routes.
- One-way roads are a powerful tool for controlling the flow of traffic. Additionally upgrading a given road to one-way doubles the number of lanes available to traffic flowing in the intended direction.
- For freight, the Heavy Traffic Ban policy is another way to distribute traffic. This policy prevents industrial vehicles from passing through the zone, forcing them along other routes.
The stadiums from Match Day, Stadiums: European Club Pack, as well as the Varsity Sports arenas from the Campus expansion all generate a significant amount of additional traffic when games are played. Ensuring good public transit and walking paths are available in these areas can reduce the load. The Come One, Come All policy makes public transport free on match days.