Traffic is the most important part of modern city builders, especially starting with SimCity 4. It's what will make or break a city and will determine the vast amount of your success in creating a working happy city. The reason is simple, cims (the people who live in your city) must get from point A to point B in order for anything in the simulation to work.
In older SimCity games, traffic was a binary flag, you either had road access or you didn't. In later iterations, up until SimCity 4, traffic modeling got more realistic, so much so that SC4's first and only expansion dealt mostly with additional new traffic features.
This guide is designed to be easy to read and short, so that you can get the basics in order to safeguard your city against some expensive errors later down the line and to understand some of the key mechanics behind traffic flow. Traffic is ever so complex and requires studying movement of various cims throughout your city, smart public transportation choices, and lots of money which you won't have at the start or even mid-term.
Cims start at their house, after they move in, and from there need to work and shop. They will drive the shortest possible distance (not the most efficient) between point A and point B. The longer it takes them to get to their destination, the more unhappy they are, but more importantly the odds of a traffic jam occurring causing unhappiness, noise, and pollution.
Sim traffic is a science. These tips are suggestions to help get started and for novices / intermediates of the game. Long time Sim fans engage in mods / cheats / addons in order to make traffic work since proper simulation of traffic and having tons of options / depth have yet to meet. Additionally, various bugs will probably always exist in the traffic system. Right now, four-lane roads are suggested to be inefficent, however, it's important to note that these are updated with each patch.
Understanding the Evils of Congestion
Traffic and congestion are two different things. Traffic causes noise, congestion causes delays. When a lot of cims are on a road, it makes the road lower land value (because of the noise). When there is a lot of cars sitting on a road, it creates noise. If they are literally sitting, then it creates delays, which increases unhappiness and odds that cims are going to move out.
One-Way Road Solution
The only important thing to really worry about in a cim's route to work is understanding that anywhere there is a traffic jam, it's going to be a bad time. The easiest way to make sure there isn't a traffic jam anywhere is through using only one way roads. However, it's time intensive because you need to make sure that you're not extending their trip by a large margin and a lot of things don't get along with one way roads, like city services. One-way roads also need a lot of intersections, but thankfully they don't produce traffic lights (see a silly example below). Cars still have to stop at intersections, but first come first serve and it's to merge into only one direction of traffic (and cims usually move over to the other lanes to let others on).
My suggestion would be to use one-way roads as much as possible, because utilizing them will lead to a traffic free city - there is rarely traffic on a one-way road except where it intersects a two way road (spawning a traffic light). This is less important in residential, where there is less traffic, and much more important in industry (where there is a lot of traffic coming from and to the highway or cargo terminals).
Using two one-way roads offers double capacity in both directions, however, it works like a highway and you need to be able to move people from one lane to another. That's why building blocks (squares) out is usually the best method, however, it can become very complex and opinionated on how to do it properly. However, one-way roads are a critical tool to help stop congestion before it starts.
Long Stretches of Road is As Bad as a Billion Traffic Lights
When there is a lot of road in one direction with stops on it, then there is going to be A LOT of cims. This is going to cause congestion. You want smaller blocks, even with intersections, so that traffic disperses across other roads.
Commercial and Industry Need the Highway
Commercial and industry want to be very close to the highway while residental wants to be far away. Consider supply shipment traffic to be the heaviest (from industry to commercial, via highway usually). Residental traffic is mostly people going home from shopping or leaving out to work. So put industry / commercial closest to the highway so that deliveries get off the exit, deliver, and then get back on without clogging up residental areas and causing noise.
Money is King
You're going to have traffic starting out because you won't be able to afford massive capacity and crazy configurations for getting on and off the highway. It sucks, but it's life, and you can deal with it by building small and waiting until you have some cash before building an expansive city.
Outside of busses, everything else is a money and operating sink until later in the game when you start to see $10k a week. Ideally, in a perfect city, you'd only run busses to keep down on unkeep costs. If you're wanting to build a rail network, create a custom map so that the tracks are free, same thing with highways. It's a lot better to have track placement where you want to build it then it is to try and earn the money to put the tracks down. As a bonus, any piece that's part of the map is maintained by the regional government (not taken out of your taxes).
Highways Can Clog
Placing the on and off ramps can cause major congestion. The intersection is the easiest way to properly pull CIMs off the road without causing a twenty mile long lineup to get off on an exit. If you ever notice a line of cars getting off somewhere, you need to fix it. The easiest thing is to see if there is a way to increase exit routes into that city area or if you want to force your cims to use alternate forms of transportation like rail or the metro.
Later in the game you can use high capacity roads to develop off-ramps that pull from multiple directions and use height to their advantage. At the start, the basic intersections are probably going to be your best bet (and they're free in the map editor).
Use Height to your Advantage
You can stack some roads on top of other roads or avoid intersections all together (which is great if you're building an off/on ramp). Don't forget you can build up and down with roads.
Use the Asset Tool
You can custom make specific road configurations to your liking. This can help replace a lot of the stock junctions and intersections with something a bit better with more capacity.
Intersections are your Worst Nightmare - Roundabouts are King
As mentioned above, intersections are your worst nightmare, however the Large Roundabout is king because it gives you a four way intersection WITHOUT any stop lights. Roundabouts are boss and you should use them liberally, since they can massively help reduce traffic. You can use roundabouts everywhere, but you should stick to the large one since the small one can get clogged with any heavy force, which reduces the benefits.
Roundabouts relieve a lot of traffic because unless they're full, cims won't have to stop. Which is just boss in so many ways. Intersections are the devil and these make it better.
Oh and don't be smart and try to draw your own off-ramps on the highway, that's how you can clog them up bad. The default intersection asset is a great start in allowing sims on both sides of the road to get off WHICH you have to do, otherwise they will all try to turn around and enter from one side only, which means you have twice the traffic in one lane.
Don't Forget Services Love Roads
Garbage trucks, schools, etc. all use roads and they put a large amount of traffic on the road. You can control where they go with one-way roads and strategic placement. Certain assets like elementary schools have a very short range, while colleges have a huge range. So think small and large and the various traffic patterns.
Oh and if traffic blocks a fire station then a building will burn down, even if coverage is highlighting it. Watch out for that gotcha.
Build Big and Small
To afford your city, you'll need a mix of small and large roads. Smaller roads should feed into larger roads in theory, but in practice the four-lane is way better all around than the two-lane, outside of its additional noise, but it's no replacement for one-way two-lane roads that are properly setup.
While I've said a billion times intersections are bad, one of the cool things is that since a two-lane one-way road has the same capacity in one direction as a four-lane road, intersections and red-lights aren't as impacting because more traffic can move through the roads at the same time.
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