This interview is part of our Road to the IGF series.

Mini Motorways is a game of designing roads to keep traffic flowing in a busy, bustling city. Through managing roads, roundabouts, and traffic lights, you can hopefully get everyone where they need to go with a minimum of irritation. The people of this world will happily let you know if don't.

One of the creators of the IGF Excellence in Audio-nominated title, along with its composer, spoke with Game Developer about the work that went into creating a procedural soundtrack for this world of traffic guidance, the nuances of making audio that calms a stressful experience, and the draw of involving the player's actions in the creation of the music.

Who are you, and what was your role in developing Mini Motorways?

Peter Curry, co-founder of Dinosaur Polo Club: I’m Peter Curry, co-founder of Dinosaur Polo Club. I’m a programmer and the vision holder on Mini Motorways. In our studio, a vision holder is the person who holds the responsibility for communicating the ideals and values of the game, and they help the team ensure that all decisions work with our vision and goals. Technically, the vision holder is also the person who has the final call on complex player-facing decisions, which could sound quite dictatorial, but in practice they act more as an arbiter. 

We’re a very collaborative studio and we encourage everyone to be able to give feedback or share their thoughts, but we’ve also found it useful to have someone with the final responsibility of bringing things to a conclusion if we can’t find a consensus.

What's your background in making games?

Curry: My brother and co-founder Robert and I have always been making games together. The earliest example we can think of is when we made a Hero Quest/World War II strategy mash-up that played out over the entire house. We used masking tape to carve up the different rooms into countries. 

We got into Doom and Hexen map creation during high school, but didn’t get started into creating our own video games until after university. We both worked as programmers at Sidhe Interactive (now PikPok) for a number of years before starting our first, short-lived, far-too-ambitious studio with another programmer in 2006. We went our separate ways a couple of years later, and game development moved to the back burner for a few years. It wasn’t until we created the game jam version of Mini Metro in 2013 that we really got back into games, and the rest is history!

How did you come up with the concept for Mini Motorways?

Curry: After the successful mobile release of Mini Metro, we started to explore what our next project would be. We spent time prototyping other concepts that we felt would work with the "Mini" lens applied to them, and we used this time to really analyze what made a Mini game a Mini game. We knew that Mini Metro worked as a whole, but we didn’t know which parts had to remain and which we could play with.

We eventually settled on cars and roads as the overall theme—the evil, congested twin of Mini Metro’s sleek trains and subway lines! One of our early inspirations was Justin Smith’s game Freeways and, like Freeways, some of our first prototypes used splines. However, after many, many prototypes, the design ended up much closer to Mini Metro than we anticipated. One of the core programmers on the team, Tana Tanoi, gave an excellent talk about the prototyping at GCAP. Through this process, we learned a lot about what makes Mini Metro tick, and developed a clear internal language that will help us as we make future Mini games. 

What development tools were used to build your game?

Curry: Well, we built Mini Motorways in Unity, and actually made good use of it this time! For Mini Metro, we were a team of mainly programmers and used it primarily as a renderer. Over the years, we’ve grown to a bigger team with people across many departments, and we now use a lot more of the features Unity has to offer. Our art and design teams also work in Maya, Illustrator and Photoshop, and Google Suites.

We learned a lot about the challenges of long-term maintenance from Mini Metro, so when we started working on Mini Motorways we intentionally invested in internal tools to help us support the game for years to come. For example, we built the simulation so every game can be recorded and deterministically played back in the Unity editor. When a beta tester encounters a bug, or an awkward building configuration, or anything else they think we need to look at, they can submit the record of their session to us from inside the game. We can then trace through the code and see why the game played out as it did.

We run a continuous integration pipeline running on Buildkite (which I highly recommend!), which generates builds on all platforms for every commit, and can deploy any build over-the-air to iPhones and iPads, or to a storefront. This is a system we’re continuing to invest in because we’ve seen how valuable this is, and how much time it saves.

According to a previous interview with Game Developer, you mentioned that you knew you wanted a procedural soundtrack for the game. Why was that? Why do you feel that best suited the experience?

Curry: One of the original design concepts behind Mini Metro was that you could see how well your network was running at a glance—no submenus, no hidden statistics, no scrolling around the map. We wanted the audio to be another vector of information available to the player. Ideally, you could hear whether your city was working or struggling and about to break. Robert and I didn’t really know anything about composition though, so it was just a high-falutin’ idea. We had no clue if it was even feasible, but we reached out to Rich Vreeland after seeing his January demo. The results speak for themselves! He turned our little game into a musical instrument.

It was an easy decision to approach Rich again for the audio for Mini Motorways. Unfortunately, given the timelines on Mini Motorways and Rich's other commitments, we didn’t get to work as closely together as we did on Mini Metro, which I missed. During the core period of Mini Metro’s audio development, Rich actually worked alongside us here in New Zealand for three weeks, which was amazing! Rich is inspiring to work with, and he throws himself into every project he signs up for and does a truly fantastic job.

Now that I’ve hyped him up appropriately, I’ll hand over to Rich to speak much more eloquently than I ever could about the intricacies of the Mini Motorways audio systems.

What processes and ideas go into creating a procedural soundtrack? Into creating pieces of music that sound well together and can be put together by the systems and player actions?

Rich Vreeland, Disasterpeace: It can be pretty overwhelming, honestly! Like any piece of music, there is an underlying structure waiting to be unearthed. After playing the game a bit, I could start to see how long the music should feel introductory for and when it should develop and ultimately reach some sort of apex.

The gameplay essentially serves as the music's conductor and, in that way, it does most of the work. It's just a matter of tying into the right gameplay conditions. The caveat on Mini Motorways is that the system is quite extensible and customizable, and so every city ended up housing different experiments. In some cities, the music reacts to you connecting roads, while in others it reacts to the nighttime or weekends, and perhaps your overall progression. In some cities, the music doesn't pay much attention to you at all; it just has itself a bit of a jam on its own. I was inspired to make the design far more asymmetrical than what we've done before. In these ways, Motorways ended up being sort of like the funky, weird cousin of Mini Metro, which I think in a way is also true for the game itself.

That said, I did try to follow the same general design approach that I used for Metro while also identifying areas where we could improve things. For instance, Motorways can generate any groove you want, while Metro is mainly limited to uniform patterns. This was a strength of the previous game, to be fair, and in opening things up a bit to a looser set of limitations, I made it a little more challenging for myself at times to reign things in, find a cohesive sound, and avoid unintended side effects. It's a delicate balance, but ultimately the wild west nature of the design makes the system a lot of fun to work with.

In the beginning, I started with some broad concepts that I thought could work, including musical analogies for the different essential objects of the simulation - in this case, primarily cars, roads, and destinations. As an example, I initially tried giving each vehicle its own constant pitch, but when your road system is flush with cars, it could get really abstract and overwhelming, a bit like the sound of the band Sunn O))) (i.e., drone metal). It wasn't really the vibe. I eventually settled on something that is just a tad less 1:1 than what we did for Metro—that is, the music is less tightly coupled to the elements of the city in favor of a more varied and stylized sound.

What thoughts went into creating the audio & sound effects for a relaxing, yet complex experience in directing traffic? What made these sounds feel like they suited the mood and feel of the gameplay?

Vreeland: Over two projects now, we've established an approach with audio that essentially seeks to tone down the stressfulness of the gameplay. Aesthetically, I think our games strive to be inviting despite their occasional plate spinning nature; the nightmarish city hellscapes you can build in Motorways benefit from a soundscape that isn't entirely on the same level. It made sense to re-use many of the sounds from Metro and find ways to recontextualize them. 

Of course, there are also new sounds like the car horns, which were designed to only be slightly out of tune. They're controlled by a hefty set of conditional parameters to make sure they behave somewhat realistically and are only as annoying as is necessary to alert the player of growing discord in their city. Many years of driving helped me dial in some of these parameters. Each driver has their own generosity score, and it includes things like tailgating forgiveness and time waiting at a red light.

What thoughts went into turning the various sounds of traffic into something melodic? Into making music from the sounds of cars and driving? What ways did you have to play with these sounds to coax a kind of subtle beauty out of them?

Vreeland: The trick on display here is that the musicality is mainly created apart from the traffic. The scheduling system that the game uses to send cars to destinations is what drives things. Musical sequences are tied to the different colored groups of vehicles and buildings, and these evolve during gameplay. I initially tried a more tightly coupled system where the cars generated the music, but this proved unwieldy. In Metro, this approach worked because there were only 7 train lines at most. You can obviously have hundreds of cars in Motorways, so I needed to think of something else.

For this project, I wanted to push the limits of our audio engine to do more. So, we added new features like reverse sample playback, non-uniform sequencing (to create groovier rhythms), and audio rate effects such as portamento, vibrato, and tremolo. We also incorporated Unity's filtering, delay, and pitch-shifting effects, which helped push our shared sound palette a bit further than Metro.

In addition, I built a system I call "The Common Tone Chord Network," which makes harmonic choices using a database of chords. It will pick a new chord based on how many pitches you want it to have in common with the previous chord. In Metro, you had to write out a sequence of notes for each city to use. In Motorways, you just pick some chords and properties upfront, and then the harmony is handled for you automatically. The caveat of this system is it doesn't always find a suitable candidate - sometimes there just aren't any given these types of rules - but we do keep it in check. Even though it has to use fallback options from time to time, it still works pretty well, aesthetically.

What ideas went into making the audio for the player's actions? Into turning traffic guidance into something that was appealing based on the sound alone?

Vreeland: Integrating the player's sounds into a soundscape like this is tricky. The game has a sequenced, music-oriented sound, but the player's interactions need to feel tactile and immediate. Many are pitched to match the system's current musical state to help them integrate with the soundscape.

What do you feel this kind of procedural sound adds to the play experience? Why draw the player and their actions into the creation of the music?

Vreeland: I love the idea of a miniature setting full of life and minor details, and I think that's where I tend to start from a creative standpoint with this type of game. Though it can be hard at times to know when to stop.

There are many little things in Motorways' soundtrack that I imagine most people will never notice. It's a labor of love and personal curiosity for me to work on a wild system such as this game has. That being said, I do think that what you may lose in minutiae, you can make up for in creating lots of variability from experience to experience. No two playthroughs sound quite the same, and there's something just endlessly fascinating about applying randomness and all of these conditional rulesets to music. When it's working really well, it starts to create the illusion of a living, breathing environment with its own sort of intelligence. I'm fond of that.

This game, an IGF 2022 finalist, is featured as part of the IGF Awards ceremony, taking place at the Game Developers Conference on Wednesday, March 23 (with a simultaneous broadcast on GDC Twitch).