Traffic congestion control

Why traffic congestion matterslink

Simutrans is a transportation simulator, so the player has to face certain challenges when building a transportation network: traffic congestion up to gridlocks and deadlocks, lack of capacity, overflowing stations, faster vehicles being thwarted by slower ones etc..

Traffic congestion and transport density/capacity can be controlled by different concepts and methods (general ones first, then the ones for certain way types).

Simutrans is a transportation simulator, so it won't show the same behaviour as the real world. Certain concepts are not possible, additional tasks may show up, but there are also things that would hardly be possible in reality.

General Conceptslink

Routing and Waypointslink

Vehicles automatically choose their route from one stop to the next one, taking into account the (negative) effects of curves and slopes. This is normally quite comfortable to the player, but sometimes leads to unexpected results, e.g. after a network layout change. To enforce a certain route choice, e.g. to equalize utilization of the network, add waypoints to the vehicles´ schedules: instead of a station, click on a tile that contains the matching way type, when editing the schedule.

Note: waypoints may not work well if placed on curves, diagonal track, crossings or bridges. Whenever a station/stop is placed on a tile that is already used as a waypoint, it will become a station/stop halt. Vice versa, a station halt becomes a waypoint when the station is removed.

Hub Station Conceptlink

Hub stations are stations that connect same or different means of transport. Passengers can switch to another vehicle. Goods can be unloaded and reloaded into another vehicle. Switching at hubs and even the choice of a route through the network are done automatically, so you have to provide just the means of travelling.

The main use of hub stations is to join a larger number of low-volume lines with a few main lines. Example: local busses and trams connecting all parts of a city to an airport or a central train station.

Hub stations can be connected to each other by a back-bone, which can actually have any shape that is needed, like a ring or a star (then with a dominating hub in the center).

Ring Conceptlink

Roads: To avoid the dense traffic in a city's center, you can build by-passes or rings, and use [[waypoint]]s to lead your vehicles onto those.
Trams: Unless you are ready to install detailed signal controls, it may be easier for you to build tram tracks in uni-directional rings (inside of a given city), which should meet each other at least at a hub station.

Crossings vs. Bridges/Tunnelslink

Crossings (between same or different way types) don't need any additional space and money to build, but in heavy traffic, they can cause serious traffic jams and delays (esp. with the new level crossings of version 99.12). Bridges (quite costly, pak-dependent) don't have that disadvantage, but vehicles will be slowed down when having to master an overpass (that is, climb one level and then return to the previous level). Tunnels are really expensive (also pak-dependent), so use them mostly for highly profitable routes, and to avoid big landscape changes or large detours.

Vehicles with Different Speed/Powerlink

Due to the permanent maintenance cost of infrastructure (and also its space requirements), it makes sense to share roads and tracks between many vehicles. But at some point, faster vehicles (like passenger trains) will suffer from being stuck behind slower traffic. You can separate traffic of different speed (or acceleration) by using waypoints to choose their routes (so you can still keep an interconnected network), and keep freight vehicles on cheap track/road with lower maximum speed.

Other methods are:
  • Prohibit slow road vehicles (even city cars and AI vehicles) to enter certain road sections, by guarding these with minimum speed signs.
  • Divert slower trains to tracks or platforms parallel to the main ones, so they can be overtaken.
  • With rail, electrify the routes used by the slowest trains, and make these electric. Then you can use the overhead cables to force these slower trains onto loop lines (slow running lines), so that express services can overtake on the non-electrified main lines.

Vehicles (of the same line) queueing uplink

If you set up lines with several vehicles (mostly for passengers/mail), you will notice that they start to queue up, instead of using a regular time or distance pattern. This is caused by the vehicles's weight increasing with higher utilization, thus making them slower, while following ones, which pass emptied stops with a higher probability, will catch up. As there are no timetables or other direct means of controlling the period or distance between vehicles of a line in Simutrans, you need some work-arounds, which also may have other benefits:
  • Instead of a single line with many vehicles, create more lines with different schedules, which meet at high-demand stations and hubs. This also allows you to adapt coverage and capacity to the actual needs, and to separate vehicles of different speed/acceleration.
  • In some cases (e. g. if there´s only one scheduled halt where the vehicles are loaded, which applies mostly to freight, or if you can rely on a certain passenger generation rate), you can use load percentage at one stop (see also next paragraph) to avoid empty tours.
  • In some cases (low-traffic roads, e.g. a line endpoint in the outskirts), the choose sign can be used (even on the way to a single-tile stop) to enforce a minimum distance between vehicles that pass it: it´s the distance between the choose sign and the stop.

Load percentagelink

Setting vehicles to wait until loaded to a certain percentage of their capacity (done in the schedule) is common practice, in order to maximize their utilization and therefore income. However, doing so often requires a dedicated loading area for each vehicle (exception: ships), or if you queue them, a dedicated loading area for each destination and freight category, and dedicated space (road/track sections) for the queued vehicles. The loading area needs to be a terminal station or a halt in a dead-end road/track section, so passing traffic won't be blocked.


City Carslink

City cars are one of the main reasons for gridlocks, especially in cities. They are created automatically, to reflect people who can't travel (to their respective destinations) by public transport. To reduce their number, you need to provide a high level of passenger coverage for all inhabited areas, tourist locations and industries on the map. If the traffic situation in a city is already too bad to have buses operate there, use trams or [[Underground mode|underground]] trains (they don't get stuck in road traffic). You can delete a single city car manually, and they also disappear after some time.

Road Vehicles belonging to AIlink

They cause similar problems like city cars, with some differences:
  • you can't delete them (you can switch to the AI player to make necessary changes)
  • they can be set to a load percentage at a stop, thereby blocking all follow-up traffic

One way roadslink

If you have a high-traffic road, you can connect it to other roads by exit-only road tiles, so city cars and AI vehicles won't enter the main road.

Traffic Lightslink

Signals can be used to control the right-of way.
Road signals control through green and red light, that switch after some time the right-of-way.


Pressing the b key will show reserved track sections in red, which can help in diagnosing stuck trains. At the same time, the tool for repairing reservations is selected.


Railway signals control the sequence of train passing, enable secure traffic and even can increase density of a very congested line. Signals may even be placed on platform squares.

All kinds of signals are initially built as two-way. Clicking to build repeatedly on the same square makes the signal one-way in one direction, then one-way in the other direction, and then two-way again. A train will not proceed past a one-way signal against its direction of travel.

Simutrans uses Path Based Signaling (PBS), which means that a train is permitted to reserve a path and proceed along this path, if the reserved path does not use any tile of any reserved path of another train. This reservation procedure happens at any stop, which can be a scheduled station halt, waypoint, or signal. Therefore, stations and waypoints seem to act as if they had invisible signals.

(Regular) signals
These are easy to understand and use: a train waits here until it can reserve a path to its next stop.

A train will wait at a pre-signal unless the block immediately adjacent to the presignal is empty and the signal at the far end of that block is green. In other words, a train in block A will wait at the pre-signal entry to block B until B is empty and the signal for entry to block C also shows green.

Long-block signals
These are used to allow single-track sections with several stations in one line, which can be passed in both directions. As reservations are allocated from one stop to the next one (see above), without looking ahead any further, sections like these would normally lead to deadlocks. Long-block signals support in avoiding these deadlocks: a train will only proceed if the entire track section up to the next signal is clear of existing reservations. (Note: the reservations are still done the normal way - from stop to stop.) In general, it is necessary to put long-block signals at all entry points of such a section, and trains/trams must not change directions in such a section (exceptions exist).

Platform Choose signals
A train at a platform-choose signal will choose a route to any free platform at its next station (in the schedule), and it will even try alternative routes to get there. If there is no free platform at that station, the train will wait. If the next scheduled stop is a waypoint, the train will proceed to the exact destination (x,y) point in the schedule, without trying an alternative route.

"End-of-Choose" sign
This creates a track signpost (and not, strictly speaking, a signal) that limits the scope of platform-choose signals. Specifically, a train will not choose a platform beyond an end-of-choose sign (exception: the exact platform that is defined in the schedule). NOTE: Do not place the end-of-choose signpost on the same tile as a signal (even if the program allows you to do so), because one of the two may be ignored.

Fast and Slow Loop lineslink

Sections of 4 track main line can be created, with fast and slow running lines in each direction. Trains will always use the shortest loop by default, therefore action is required to make both be used efficiently:

On simple networks, when routes are set up, a waypoint should be set to prescribe whether the train should use the fast or slow loop in each direction, and rolling stock chosen to allow a corresponding running speed.

Alternatively, the longer loop may be electrified and the shorter not. Then either all "fast" or all "slow" trains can be provided with electric locomotives, which will use the loop to pass / be passed by steam-powered trains.


Aircraft are an expensive (per-vehicle) option for congestion control, though full loads of airplanes can very rapidly pay for their initial costs, depending on speed and number of passengers carried. Note that for aircraft to be effective for long-distance travel, airports must be well-connected to the city centres and attractions of a map.

If the airports are off on their own, without direct, fast linkages to the city center, it will be difficult to generate the numbers necessary to make the planes profitable.

In some games, it is possible that your rail service will still carry more passengers than the airplanes, so plan ahead to make sure sites for airports are as close as possible to the city center. Though a good high-speed rail line will be able to get closer to the city center more often than not, because airport infrastructure tends to be very land-extensive.

Originally written as http://en.wiki.simutrans.com/index.php/Traffic_congestion_controllink-external

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Contributors to this page: Frank and system .
Page last modified on Monday August 15, 2016 15:50:34 GMT-0000 by Frank.

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