Wishbone: You are obviously not a programmer. Trust me, there's a big difference.
Of course, the "agents" in SimCity 5 are not well implemented, to say the least, or at least they weren't at launch. I haven't kept up with the various patches for the game.
Kristian: Care to expound on the difference between "agents" and "equations"?
Certainly. Understand though that I'm talking out of my ass here (as are most people on the internet, but I prefer to be upfront about it). Basically, I'm making educated guesses based on my experience as a software developer. I haven't seen the source code to any SimCity game, so exactly how each game has implemented each function is not something I know. I can tell you the difference between the two methods as I see them though.
Determining traffic by equations means calculating the traffic load on any given stretch of road based on a set of parameters, such as population density, time of day, workplace density, etc. All in all, this results in a single figure (or two, one for each direction) for any given stretch of road for any given time that the game updates the road traffic. This is quite fast to do, but also means that the traffic you see in-game is, for lack of a better word, "fake".
Using agents means that the traffic load is not calculated as such, but is an emergent property of the simulation. Instead of performing a calculation of the traffic load, and then representing that amount of traffic visually in-game, each person in the city is represented by a so-called agent, a small program-within-the-program, which keeps track of where each person is at any given time, where they're going and how they are going to get there. With each iteration of the game loop, the position of each agent is updated, and calculations are made to determine how the agent should progress. Each agent is represented visually in the game at all times (or at least when they are out and about, probably not when they are in a building), so the traffic you see in the game is not "the number of cars placed there by the game at this time because at this time there should be this many cars on the road", but rather the actual cars used by the agents to get where they are going while obeying the laws of traffic. This is a very calculation-intensive method of doing it, but it is also potentially much more accurate, because it does not depend on the developer's estimates of which factors affect traffic and by how much. Of course, from a technical point of view, there are many different ways to actually implement the agents, and from what I'm given to understand, the implementation of the agents in SimCity has (had?) any number of aggravating flaws, both performance-wise and logic-wise.
