Why road-building in Cities: Skylines is a pleasure

This is The Mechanic, where Alex Wiltshire invites developers to discuss the inner workings of their games. This time, Cities: Skylines [official site].

Cities: Skylines is a game about building roads. Its lovely set of road-building tools allow you to scribe beautiful curved boulevards into the gentle slopes and combes of virgin lands, and it has inspired 19-page forum topics entitled Show Us Your Interchanges and Steam Workshop lists 24,482 interchange designs.

Oh, and an incidental byproduct of a good road system is the growth of a city around it.

Cities: Skylines is a sim that feels uncommonly alive and reactive to your planning. You can watch each citizen make its way through your creation, from home to place of work, and from there to visit a park. They may walk or take a bus or train, but you’ll mostly notice them driving, creating traffic which fundamentally represents the health of your city, a pulsing network that’s driven by Cities: Skylines’ beating heart:

THE MECHANIC: Traffic simulation

Developer Colossal Order was no stranger to transit when it started making Cities: Skylines. It’d already made two Cities in Motion games, which are about building public transportation systems. But for a game about managing a city, they needed to capture its essence. “Roads are part of the character of a city,” game designer Karoliina Korppoo tells me.

The team also wanted to achieve something rather tricky, to feature deep micromanagement-based play that would also appeal to less experienced players, and they realised that the realtime, flow-y nature of traffic could be the key. “It seemed like the one thing where constant quick changes felt good and it felt meaningful for the player to make changes to the road network,” says Korppoo.

They also wanted the city to feel alive with residents so players would feel attached to what they’d constructed, and so every citizen has a home, workplaces and family, and is simulated as they move around from one to another. Materials needed for commerce and industry are modelled moving from place to place, too. And once they’d built that system out, it was only logical that they’d extend it to the traffic, since it reflects all these movements.

“We do not like faking citizen behaviours,” says programmer Damien Morello. And thus you can watch cars purposefully drive around the city, showing off what’s running in the simulation itself. For the player, that means roads are absolutely necessary for linking each part of the city to the rest so the citizens can access them. After all, the city is not statistically modelled so that as long as amenities exist, they’re factored in. They have to be connected, and more than that, how they’re connected matters. There are a few circumstances in which the game does fake things, however. If a traffic jam gets completely out of control, the game will remove cars to avoid perpetual gridlock.

Surprise: Cities: Skylines’ traffic simulation wasn’t easy to develop. But not because it’s a difficult system to model so much as it was difficult to present in a way that players can understand the emergent complexity of thousands, and even hundreds of thousands of journeys.

In other words, Colossal Order’s early attempts at the traffic simulation initially lead to chaotic traffic jams that were hard for players to diagnose and therefore not fun to fix. The solution was to simplify citizens’ decision-making processes and the traffic rules, teasing out the causality so players could see how congestion was happening.

So what are Cities: Skylines’ traffic rules? The first rule is: you do not rear end the car in front of you. Programmer Antti Lehto is pretty emphatic about this rule. The second rule is: you don’t exceed the speed limit. Third rule: you stop at red lights. The fourth rule says that only one vehicle can be in an intersection at any one time. The fifth (it feels like this could maybe be prioritised a little higher?) is you don’t run over pedestrians. The sixth rule is that the first vehicle to decide to go to a given location is served first.

And the seventh and final rule is that these don’t rules apply when the player isn’t looking. This is a rule that is there to save CPU time, but even here, the game only skips rules that don’t have a major impact on the simulation.

The result is a game which runs according to the efficiency of your road system. Since your citizens and goods are moving around the city, the time they take to get to their destinations directly affects the speed of growth and change. “If your commercial areas are not getting goods to sell, they will eventually run out and if none arrive within a reasonable time, they can be abandoned,” says Korppoo.

The simulation has to react to the fact that it’s such a dynamic city, with buildings developing and becoming disused, as well as the player making direct changes to road layouts. But vehicles only recalculate their routes when they get close to a modified area. As you’d expect, it’s a solution for dealing with change that saves CPU time, but it also gives an extra sense of life to the cars. As a driver in the real world, you won’t necessarily know a shop has shut down or the road you’re using has closed before you get there.

All of this means that the other game mechanics in Cities: Skylines need to account for the fact that the time for citizens and goods to reach a given destination is unpredictable. All buildings spawn with their stocks of goods half full, so they won’t immediately go into decline before deliveries can reach them.

“Balancing is hard work and we spent a lot of time checking what kind of resource buffers felt right,” says Korppoo. “This is part of the core of the game, so while it was difficult to get right, the whole game would have felt empty without it and all that work was worth it.”

The effect is a game that continually nudges you to focus on roads. Every development starts with one: you can’t zone until asphalt is down. And improving the city by raising density and value is as much a matter of honing your road design, optimising routes between areas and amenities to be as short and clear as possible.

“Judging from Steam Workshop, there were and there still are optimal road layouts which are beyond weird,” says Morello. “But the simulation is realistic enough to allow real world layouts be optimal.” And so an official interchange design, the single-point urban interchange, which has been widely used in the US since the mid–1970s, is a mainstay for Cities: Skylines players, too. But few designs can hold up to the diverging diamond interchange, which is horrifically complex to build but shrugs off heavy traffic in a tight space.

“That said, the goal ultimately never was to mimic a real city road network, as the game is aimed to many kinds of players and both the ones who want to play optimally and those who wish to create beautiful cities need to have fun,” says Korppoo. “So the game focuses on flexibility with the road systems so that many different solutions can work.”

And the flexibility of roads in Cities: Skylines is its greatest joy. The building tools are simple and surprisingly expressive. Building curved roads is a pleasure, and assembling a functioning interchange holds the deep satisfaction that comes with all the learning and planning you have to put into it. Perfectionists might howl at the ease with which an attempt to build a perfect grid can turn out looking decidedly organic, but it feels like you’re laying down a place, with all the imperfections that come with that. And there’s something magic in watching little computer cars negotiating the bumps and turns you’ve set, by design or happenstance.

“City-building games are slow-paced and some of the time playing is spent on pondering what to do next or waiting for money or population to accumulate,” says Korppoo. “This is the time to just stare at an intersection and enjoy it.”


  1. GDorn says:

    One feature of Sim City 2k and 3 (and maybe 4?) that I miss in Cities: Skylines is the ability to build a city without roads. It was possible to have a vibrant, profitable city using only rail and subway.

    I get that it’s somewhat unrealistic for an entire city to have no roads, but real life cities have no-driving zones (shocking to most Americans). Sometimes I look at the worst intersection in my city and wish I had a way to ban driving there entirely.

    • haldolium says:

      And even in the cities with the worst possible infrastructure for vehicles, the economy doesnt suffer since there is always a way (literally)

      I think Ill go back an try what C:S is offering by now, but the sheer focus on proper road placement (even though its been one of the best as such) undermined the thought of building an actual *city* (you know, these things that never work out when it comes to traffic)

    • Landiss says:

      In general I think the game was based more on how American cities look.

  2. chudbabies says:

    Yess-s, Cities: Skylines!1

    Each time I pick up a “civilization building,” game, I either fail at combat (boring!), or try and jump straight through to a future city world where all people in cities live like hobbits in San Francisco. Failing this, I become frustrated and begin building hovels in a Dwarf-clone. In Cities, I build roads and become frustrated late game tools are not available at start, but press on and lose sight of my beautiful town and mayor gigs, never quite planting that high school…

    Now, I find myself imagining how actual townships would develop working simulations with simple point-and-click terraforming abilities, which they print out a .gif of and post in the basement of City Hall six months prior to any construction work on demolishing a person’s home! (that’s the part that scares me the most) All the human-dwelling simulation program and modelling service would need is an agreed upon physical rules set, like 3-D printers. I still dream of hobbit homes tucked away in hills or planted with deep forests and garden patches all over the city center.


  3. dirtrobot says:

    Zzz wake me up when they finally hit 30000 interchanges.

  4. Blad the impaler says:

    I truly love the game. Just one of those guys who likes to see what kind of chaos I can construct. Fantastic community with lots of talented contributors. It could be nice to have some sort of merge lane system – and perhaps a bit more common sense from your peoples as they navigate your cities. Where traffic is concerned you must speak by the card, or equivocation will undo you.

    • ColonelFlanders says:

      Traffic Manager: President Edition has a behavioral change to the traffic AI, and it does almost exactly what you want.

      • Rochambeau says:

        Exactly. So many great Steam workshop options including Traffic President.

  5. Samudaya says:

    Traffic management killed the game. The expansions didn’t sell at all. So even if the traffic was fixed it was way too late and nobody cared anymore.

  6. c-Row says:

    That header image makes me wish I could better supress the urge to build everything in grids.

    • Daemoroth says:

      Oh hell yes, I know what you mean! I always see these gorgeous, flowing cities and hate my incessant need to set everything to a grid!! :'(

  7. Landiss says:

    For the game was fun for a while (I have a dozen hours logged in Steam). The problem for me was that the same thing that made it fun, the traffic simulation, was what had glaring issues visible after you spent some time with it and there wasn’t that much strategy outside of that part. Of course for many people the big appeal was to just build nice things, but it didn’t hold me. I need more strategy and for that the game’s traffic system would have to be more detailed and at least a little more realistic.

    • Rochambeau says:

      Realistic? It’s realistic enough that there are European countries that use Colossal Orders software to preview new roads and upgrades to current transportation to see how it will affect their citizens.

      So while you may find it frustrating or too challenging, it’s definitely as close to “real” as you’ll get in a game.

      • try2bcool69 says:

        I’ve driven 100,000+ miles per year for the last 13 years, and there are several basic traffic behaviors that are glaringly omitted from the game, that make it really aggravating to proceed beyond a certain point unless you know how to counteract them WELL before you start.
        For example, you lay down a 4 lane highway with turn lanes, people will sit in the line for the turn lane, no matter how many intersections down they are actually going to turn, instead of using a through lane until they pass the last intersection before the one they actually want to turn at. If one car does that, no problem, when EVERY car does it, it’s ridiculous. Through lanes remain lightly traveled, while the turn lane gets clogged and backs the whole system up. It’s unrealistic, and it happens in other situations as well.
        Also, in real life, if a road is backed up, people will find an alternate route…not in CS! These idiot AI drivers know the shortest way, and nothing else. You can insert a better path, that’s a little longer, and they just say “Nah, I’m good! I’m already 7 hours late for work, I’ll just wait for a minute until I despawn back to my house.”
        Nope, this is not really a city builder so much as an overcoming faulty traffic AI with over-engineered road systems sim.

        • carewolf says:

          Yeah, that is annoying. The solution in game is to reduce the number of intersections (I often delete every other one). Or if the traffic is sufficiently heavy, turn whole or parts of intersections into interchanges (so the road splits and merges instead of having crossing traffic). It is a little puzzle in the game.

          If the AI is REALLY stupid in one place, just bulldoze that place and see how the traffic splits ;) .. Also cars on bulldozed roads magically fly home. It really clears up traffic :D

    • the poison king says:

      People who bemoan that lack of realism simply need to git gud.

      Okay, okay. That’s a bit mean. But the traffic problems always have a source. It’s not perfectly real because it’s not a 1:1 simulation, but if you have traffic problems, it’s probably because part of your design is inadequate.

      This is the one big issue with Cities as a game. Everything seems to be going well, and then…it falls apart. It’s very easy to expand yourself into a tricky situation without realizing until it’s too late.

      • Rochambeau says:

        Well stated.

        You really do have to be forward thinking while planning. You can’t just build and expect it all to work perfectly. And even when you think you have thought out far enough to not have issues or have very few, BOOM, CS throws you a curve. Some find it frustrating, others find it part of the charm and challenge of the game and in a somewhat sadistic way, look forward to it.

        It’s all in the mind set that you have. ;)

      • Landiss says:

        I’m not sure why everyone assumed I was having problems with it because it was difficult. No, not really. The game is ridiculously easy in general, it’s quite impossible to lose. At least it was when I played it (around the time when it was published).

    • Aetylus says:

      You don’t *really* want more realistic though. In skylines there is a grossly unrealistic time split that means you build entire neighborhoods faster than you Cims drive to work. That is unrealistic by a factor of *thousands*… yet it is essential to making it fun.

      There is also a population split, which means you only have to build a few hundred houses to get epic traffic. You only have to put in the effort of planning a hamlet to generate traffic challenges to make Beijing weep. Again, essential for fun.

      Do you really want *realistic* when it would inevitably mean devoting years of your life to the game just to build up a decent volume of in-game traffic.

      • sarnoc says:

        Speaking (literally) as an expert in strategic transport planning, CS is remarkable in the level of detail. Sure, it can sometimes be really frustrating when it makes dumb decisions, but to be honest, even the most detailed professional transport modelling software is little better.

        However, its railways are absolutely terrible. I like railways, and I like good ones. I think what frustrates me about CS is that the team designing it had such a good base to develop various ‘packages’ of DLC – I’m thinking a transport upgrade package (including trams, but what about bus lanes, one way railways lines, signalling, aircraft improvements (like helicopters), a proper day/night cycle that affects travel requirements in far more detail than they have actually implemented. They seem almost to have had a mind block since they finished the game. There’s so much that could be added, tweaked and improved, and then delivered as cohesive package of set upgrades. Heck, I’d pay for more capable zoning and micromanagement of the types of buildings – and yes, I know I can do that by modding, and I do, but surely it’s the job of the developers to spot that kind of thing and help the modders with frameworks?

        • Aetylus says:

          I’m assuming they are deliberately keeping things high-level and avoiding micromanagement… because for much of the player base that micro-management reduces the enjoyment.

          Cities in Motion 2 for instance covers the sort of detail you are looking for (down to bus timetabling etc)… but to me at least it always just felt like a chore.

        • Landiss says:

          “even the most detailed professional transport modelling software is little better”

          That’s a shame, because I think technically it should already be possible to get to incredible level of detail in such professional simulations. I would think that especially advancements in neural networks should really bring great results in this area. If you are saying that pro software get to a similar level of detail and of correctly predicting traffic behaviour, as a game that can run on an average PC from few years ago, it just makes me sad.

      • Landiss says:

        Well, please don’t tell me what I want, ok? I do want more realistic. I didn’t say I want a simulator. But I want something that has proper rules of cars behaviour on intersections, for example. It was quite a while since I played it, so I don’t remember all details, but I do remember that there were glaring issues with it. Of course they were visible only because they did get to that level of detail first. For example, if I remember correctly, there was no way to create real roundabouts in the game, the way they are used in most European countries, meaning that cars that are on the roundabout are privileged. I also remember problems with slip roads and, what someone else already covered, issue with lanes changing. Not to mention things like deciding where to put lights on the intersection and how the lights should work (for example, in real life the main road is privileged and has green for a longer time, often there are sensors that detect if there are cars on a specific road or lane, same for pedestrians, on top of that you have things like green wave). Then you have things like park & ride, which, if I remember correctly, were impossible to set up in the game.

        Trains were completely, utterly broken when I played it, including underground, if I remember correctly (there was underground, right?). I hoped they fixed it since then, but, judging by the other comments, apparently they didn’t.

        I do realize that it’s a lot to ask of the game, but my problem with CS was that the game was focused on that part – and then it didn’t really deliver. Other parts of the game were pretty much a creative sandbox. And I’m guessing that’s how people treat it, judging by the kind of mods there are. Most of them seems to be adding more models for buildings etc. That’s not really interesting for me, I like strategy games, I like management games and there was not enough of that for me in CS.

        • ColonelFlanders says:

          Really there are a lot of things that have changed in the last year. The Natural Disasters mod brings a much needed ‘failure state’ to the game.

          There are about 10 mods that I consider essential and change/improve gameplay, make it deeper, or add greater customisation to difficulty.

          Traffic Manager President Edition
          Network Extensions
          Rush Hour
          Realistic Population/Demand
          Difficulty Tuning Mod (ESSENTIAL)
          No Traffic Despawning
          Better Bulldozer

          And many others that I’ve surely forgetten. I have about 40 mods installed, all of which I find immensely useful and improve the game immeasurably. It would have been nice if CO added these things themselves (not least to improve loading times), but the fact they exist and that there is modding aupport is a fine thing.

          • Landiss says:

            Ok, thanks for the list. Perhaps I will get around reinstalling the game and trying it with those mods.

    • Dogahn says:

      I get you. The game could use more… well, game. Where’s the challenge, the struggle, the drive for innovation. The game’s built in goals are more like milestones congratulating you on expanding the systems; not circumventing or planning around obstacles.

      That said, I’m waiting for the disaster expansion to drop in price as it’s my last chance offer to this game. I want chaos forcing me to adapt, an environment that forces opportunity to redesign within an established system, or an event that outright kills my city. I want to feel like I’m winning, can’t do that if I feel there is no way to lose.

      Part of the fun of SimCitys was that disaster happened and you had to deal with it. Assigning police to control riots, Deploying firefighters to control a fire, and you could lose control ending up with a total rebuild project.

  8. CaptainKoloth says:

    I love this game and building and just watching roads get used in C:S is really fun in a way it never was even in the peak of Sim City, which was 2000 (which is not to say C:S is the better game, but they certainly are in this aspect).

    Now if only Cities could do railroads in a way that isn’t totally and utterly broken. (And for that matter ships and highways).

  9. rgronow says:

    Traffic/ congestion management is be far my favourite thing sky this game!

    I have spent many hours attempting to fix congested roads!

    It’s a deeply satisfying mechanic! If there was a spin-off game that was just about managing roads – I would buy it.

    • Aetylus says:

      Ah yes… My favourite minigame… go to Steam Workshop… download random city called “Helpmytrafficisborked” or something similar… fix… upload.

  10. Dogshevik says:

    Long-time lurker here. Hi.

    First things first, I am a fan of CS. But I got a laundry list in my head about “everything wrong with CitySkylines” and alot of it revolves around roadbuilding and managing the traffic AI. The article and the devs´ comments make it sound like a wonderful experience. I can´t say I agree.

    Maybe the fault lies with me. Seeing a pretty sophisticated game my ambition to build “realistic” cities may know no bounds. Maybe I demand perfection from something that was merely meant as entertainment.

    But frankly speaking seeing a 3 kilometer traffic jam on a four-lane street, because -every single car- is using the same frigging lane (despite being offered two lanes to take that right turn they crave so much) is making my skin crawl. The same applies when you actually try to make a curved 180 degree turn that doesn´t look wonky or the zoning of two streets get into each others way, making a mess. Not touching trains. Never.

    Yes, mods can fix alot of problems. I currently use 6 just for roadbuilding. They also create new problems. Patchwork problems. Unreliable updates, compability issues, documentation etc. Also, spending half my time in the steam workshop, not to look for flashy extras, but so I can “fix” the vanilla game is not what I was hoping for. The constant supervision required is souring the experience for me.

    I waited for a long time and expected they would at least canonize the “precision engineering tool” or provide an equivalent. But instead of fixing core mechanics we are offered DLC after DLC with -more stuff-. I can´t say that plopping down yet another service building to take care of yet another new problem/need of the citizens (looking at you, heating pipes) is enhancing the experience in any way when obvious problems of basic mechanics are not getting tackled with.

    My guess is the devs are at this point just content to go the easy way. In light of their success it is understandable. In light of the game being so close to excellence it is almost excruciating.