RCI demand is the basic principle in which all city simulation games are usually built upon and requires a balance between supply and demand of products, consumption, and employment. There is a lot of additional very "interesting" and "hardcore" factors applied that you would normally gloss over, such as the huge impact of education.
First, let's talk a bit about the driving force behind RCI by breaking it down into each component:
- Residential: Requires employment, desires high land value, and dislikes noise.
- Commercial: Requires mostly uneducated employees, requires goods to be delivered from factories or to be imported, causes massive traffic on the pipeline between the supplier (industry) and itself. Residential comes here to work and shop. Produces noise.
- Industry: Requires employees, regular industry requires uneducated employees while office buildings required educated employees. Produces pollution and noise.
Okay, so here is the deal. In a cim's daily life, they move from their home to work, then to shopping, and then back home. They will take a variety of methods of transport, preferring the easiest route. Distance isn't as much of a factor as time, as a cim will get progressively unhappier the longer it takes them to reach their destination in relation to distance.
Cims will take public transport or any other major transportation network to get to and from work and shopping. Industry and commercial will ALWAYS run vehicles between each other in order to supply the commercial buildings. Industry will, in general, always create its own traffic moving to commercial buildings and any freight option (highway to export goods, freight trains / ships to export goods).
Any kind of resource that isn't available has to be imported. The danger to imports is that they can either be unavailable (the game will say that a commercial building can't be supplied) or will cause abandonment before they reach their destination (if traffic is too severe).
A huge issue with industry is that it's required in order to make sure commercial runs properly, but produces a disgusting amount of pollution. You'll want to put it in the off-skirts of town and later in the game, you can meet industry demand with the no-pollution (but high education requiring) offices.
R needs C which needs I and R needs C and I for jobs. So if you see them all low, check on your cities happiness (your city isn't attractive enough for demand) or modify taxes. If you see only one in demand, but want more of another, then you have to meet that demand before the other sectors can expand. Often times you'll run into high residential demand, low C and I demand.
No Workers - Educations Big Impact
Sometimes you'll see abandonment because of lack of workers. There is generally three things that impact the lack of workers - no workers available (build more residential, increase residential demand), traffic preventing workers from reaching their workplace, or you could be running into an education problem.
To utilize industry, especially farming, you'll need lower education workers. More accurately, you're going to need a mix of highly educated and barely educated folks to make sure your town runs right, unless you plan on importing everything and only having offices and low commercial demand. You will, essentially, need to create a slums area for cims to move into that doesn't have much education and more nicer areas with better education. There is an argument that the system let's educated workers work in uneducated fields, but a lot of evidence shows that having lower educated workers can help have a surplus of workers for factories, since educated workers should fill jobs out from most educated to least.
This can be done with some interesting school placement, as you can see the green fades away, you can let the outskirts of each district serve as your under-educated workforce or get rid of any industry that requires the uneducated, but remember - commercial will need to import and they might not want to, which will cause them to run out of goods.
That's basically RCI in a nutshell! Go out and have fun building cities!
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