Jun 20, 2016

Cities: Skylines: Highway to hell, and the Great Commerce Tipping Point

Amidst all the other gaming-related stuff I've been getting up to, I've also found the time to do some more experimenting with Cities: Skylines. Last time, I tried to create a walkable city; this time, it's time for something completely different.

I picked the Black Woods map, and decided to experiment with a city on several islands, connected by motorways. I'll use public transport and invest in walking infrastructure like before, but I do want to see what the traffic gets like if we rely on the pre-built highways to connect different parts of the city together. To make my life a little easier, I'll be preferring offices over industry.

Here's a view of my work in progress. The area on the left is where I started; I next expanded to the island in the center, and next started building on the right. The very regular grid at far right is my massive office zone.


First, offices. There are three demand bars on the user interface, just like in Simcity: the green measures residential demand, the blue commercial and the orange jobs, i.e. industry or offices. I was building some high-density residential zones, and there was a pretty robust demand for jobs. I decided to fill this by just building offices for as long as the bar stayed up. This is the end result:

The demand stayed up, I kept zoning offices, and the zones kept filling up. The weird thing is that despite all the office buildings operating, having employees and paying taxes, they create practically no traffic at all. My massive office zone was an eerie, abandoned Edge City wasteland, patrolled by the occasional police cruiser and garbage truck.

But hey, it got people jobs and paid my bills.


This is not to say that my city didn't have traffic. On the contrary. With all those beautiful highways out there, my cims sure liked to drive around.

Some of the only real chokepoints I ran into were, unsurprisingly, the highway bridges connecting the island to the mainland.

To be very specific, the problem was merging. As long as everyone stayed on the highway, everything was fine. It's when they get on or off that we get into trouble.

People complain a lot about the way cims pick lanes, and it does at times look very silly from a real-world standpoint to see a massive logjam of cars on one lane of a highway, with the other lanes completely empty. However, these complaints are mostly misguided. Cims pick their lanes perfectly logically and predictably - mostly - and it's your job to plan around that. Basically, in almost all traffic situations, you can tell which cars will end up on which lane, and you can plan your interchanges around that.

My issue was that the second exit on the western mainland was by far the most popular. The exit could handle the traffic, but this meant that a majority of the vehicles on the highway would get in the center lane until they passed the first exit, and then switch lanes to the right. The trouble was, this meant that the center lane got completely backed up, especially when different vehicles changed lanes at different times, bogging down the exit.

To me, situations like this are excellent opportunities to experiment. One of the things I tried was splitting the highway into three ramps, one of which rejoined the highway later. I call this the Acme Interchange.

Believe it or not, this kinda works! However, it's not optimal. There were just too many vehicles getting off at one ramp; even when I got them to line up nicely, the sheer mass of cars going down the single-lane ramp created a constant accordion effect; even though the line was constantly moving, every time someone braked or slowed down, a disruption would ripple down the entire line of cars, backing up traffic at the previous interchange.

The only reasonable solution is to split up the traffic among several exits. The interchange started life as a standard cloverleaf interchange, which you can just plop down from the road menu. The problem with a cloverleaf interchange is weaving: when some cars are trying to get off the highway and others are trying to get on, their paths can cross and end up in a horrible gridlock. In a regular cloverleaf, the off-ramps are before the on-ramps, creating a weaving effect. On my island interchange, I'd gone to some lengths to avoid this with flyovers and slightly more complicated on- and off-ramps.

I eventually decided that the only way to fix my original interchange's problems was to build a flyover. If I could route the traffic coming from the right of the picture onto the freeway heading down (it's not always clear where the compass directions are on a Skylines map), that should make everyone's life much easier. That's what I did, creating what I suppose might be called a slightly complex interchange.

It's certainly the most complex I've built. Effectively, there are two crossing motorways, which are connected with a commercial zone at top right, industrial zones at top left and bottom right, and a residential zone at bottom left, the top two via a roundabout interchange. It may not be pretty, and it takes a massive amount of space, but it works!


The Great Commerce Tipping Point

While I was busy building offices and interchanges, my commercial zones suddenly collapsed.

Out of nowhere, all my commercial zones started flashing "Not enough goods to sell", and started shutting down. I had no serious traffic issues, and there are plenty of external connections. This didn't matter: all of a sudden, my commercial buildings were being abandoned wholesale.

In the picture below, every empty blue zone was a leveled commercial building before all this happened.

So not only did my commercial zones inexplicably run out of goods to sell, but demand for commercial zones also collapsed. I don't know what causes this, and neither does the Internet. I obviously went over my traffic connections, looking for traffic jams and not finding any. All of the zones complaining about no goods to sell were well-connected to the outside connections on the map, which means they should have been well able to import everything they needed. Only they weren't.

Since I didn't know what to do, I formulated a hypothesis and tried it. My thinking was that for whatever reason, importing goods wasn't working. What if I tried building more industry, so I could produce the goods myself?

That worked! At first, the industrial zones also complained about not getting imports:

Eventually, though, when I got enough industrial buildings working, the goods situation went away as mysteriously as it had appeared. I still don't know exactly what happened, but I'm assuming that for whatever reason that isn't actual physical traffic capacity, the outside connections couldn't deal with the necessary level of imports. This would seem to imply that a city with no industry and offices only won't work, or requires doing something quite different. I don't know if that's true or not, but at least I learned what to do when my commerce inexplicably collapses.


With the goods problem fixed and the last of the nine areas filling up, it was time to call this a finished city, and move on.

Leaving behind a massive traffic jam of cargo ships.

This is exactly like what happens with trains: about half of those ships are traveling between two of my cargo harbors with 4% or 8% cargo on board. Why they don't despawn, I don't know. Some ships do. This doesn't actually seem to cause any problems, because it started long before my goods issues and persisted throughout them; after I fixed that problem, the river was still full of ships:

To conclude: what did I learn?

- offices generate no traffic whatsoever compared to any other kind of zone
- freeway interchanges take up massive amounts of space
- weaving is the highway killer, so think about your merges and build flyovers
- if your commercial zones suddenly collapse because of no goods to sell, see to your traffic situation, and if that's not the problem, build industry. It worked for me.
- Cities: Skylines is still awesome

No comments: