This article is considered accurate for the current version of the game.
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.
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.
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 or metro takes the journey off the road system altogether. Citizens are quite keen to take public transport if it is available, even more so if it is free.
- An overloaded public transport system can cause problems of itself, with citizens waiting many in-game days to catch overcrowded trains, metros, and buses. Also, too many buses in a small area can actually cause congestion.
- 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.
- Pedestrian 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.
- 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.