Looking further at your guide, I notice that for both the Network Speed properties and the Commute Trip Maximum Time, the units of each are not mentioned. And from what I have read, there doesn't seem to be a consensus as to what the units should be. But I've come across a relationship that should answer this question, and which should be very useful for anyone who wants to design or modify a traffic simulator.

It is well known that in the Reader, the network speeds are described as being measured in kph. Some people have disputed whether or not this is correct, but the evidence that I have found indicates that it is. It turns out that if your city is built to the standard Maxis scale (64 squares = 1 km) and your speeds are specified in kph, then Commute Trip Maximum Time is actually the time in minutes for the longest permitted round trip. It's easy to verify this mathematically.

You might think, "Wait a second! The Maxis default value for Commute Trip Maximum Time is 6, which implies that Sims can only travel 3 minutes before they run out of time!" That's absolutely correct. Fortunately, Sims never have to wait for the bus or the train, because they're always on time, and in fact, they run continuously. And the three minutes doesn't count the time the Sims use getting to their car from their house, backing out of their driveway, etc. Still, it's an awfully short amount of time, and it's short enough that all the early NAM traffic simulators tried to fix it in various and sundry ways.

Now we have Simulators A and B, which use speeds which are certainly in the kph range. But their Commute Trip Maximum Time is 17 and 24, respectively, which implies one-way commute limits of 8.5 and 12 minutes. I know that for Simulator A, the rationale for this number was that it was big enough for Sims to cross one large tile on an avenue, which, of course, it is. But is that the right criterion to use? How many people have jobs where it takes them only 8 1/2 minutes to get to work? Some, but not many. What would happen if that were the limit in the real world?

To be more specific, a value of 17 isn't large enough for Sims to drive from one corner of a large tile to the opposite corner and back, especially since true diagonals don't exist. For cities with zones placed far apart, these values are just too small for such cities to function well. Even a large tile is only 4 km (or 2.5 miles) wide, and in real cities, zones are often placed farther apart than that. I am currently building a region of Chicago to scale, and when I tried out the new simulators, the individual cities started falling apart.

So what's the ideal value for Commute Trip Maximum Time? Well, basically, it should reflect real-world commutes. I'm currently using a value of 180, which represents a maximum commute of 90 minutes each way, which is not uncommon for a maximum in many places. This has made realistic traffic flow possible in my cities, as well as greatly improving inter-city commuting. Of course, once you get past a certain point it doesn't matter how high this number is, since it's reset whenever Sims cross city boundaries. So I use the 180 number primarily for realism. However, if the Sims want to take a convoluted route from one end of a large tile to another, which would be realistic in a real city, this number makes that possible as well.