I've recently been looking more closely into the requirements for RHW and NWM compatibility for traffic simulators. In general, I think that the current settings of Simulator Z are fine here, but there's one area in which I'd like to make a somewhat significant change, and I'd like to get some feedback before I do.

One of the requirements for NWM compatibility is that the capacities and speeds per tile of roads, one-way roads, and avenues all be the same. Not knowing how NWM was implemented, I simply have gone along with these requirements up until now. But taking a closer way that one-way roads are used, both by themselves and in NWM, I want to question that particular restriction, and possibly change its implementation in Simulator Z.

In NWM, one-way roads are used only as part of the one-way road extensions; they are never used as part of two-way avenues. So any changes to one-way roads would affect only the one-way thoroughfares in NWM. So the question arises: Do these really need to be the same speed and capacity of roads and avenues? If not, what should their speed and capacity be?

In the original Maxis simulator, the speed of one-way roads was identical to that of regular roads, but their capacity was doubled. These ratios hold true in Simulators C, D, and E as well, where all network capacities are multiples of the original Maxis capacities, and the speeds are the same. There is a great advantage to using one-way roads in these simulators. In Simulators A, B, and Z, the capacities and speeds of one-way roads are the same as regular roads. Nevertheless, due to the asymmetry between morning and evening commutes over various routes, and the fact that congestion is calculated on a whole-day basis, there is a slight advantage to using one-way roads in SC4. My observations have shown that that advantage will range somewhere from 0% to 20%. In practice, this is barely enough to notice, and I have received numerous complaints that one-way roads are now almost useless, and not worth the trouble of building. To be honest, my experience supports this point of view.

What does traffic engineering say about the speeds and capacities of one-way roads? The few sources I have been able to find agree on a single number: that the capacity and effective speed of one-way roads is in general two-thirds higher than that of equivalent two-way roads.

That's a big difference from the current state of things. How does this compare to what we can observe? First, a basic principle of traffic engineering (and actually of physics) is that for comparable roadways, capacity is proportional to speed. If you double the speed of traffic on a road, then the road's capacity - the number of cars passing over a given spot per unit of time - also doubles.

Next, let's look at some RL examples. For example, here in beautiful Eugene, Oregon, which is a small city of about 150,000, there are a number of one-way roads and also wide one-way roads that travel across town. There are 11th and 13th Avenues, which are two-lane, one-way roads, and then there are 6th and 7th Avenues, which are four-lane, one-way roads. Speeds and capacities on these roads are basically the same. Especially near the downtown area, there are stoplights at virtually every block, although these thin out the farther away from downtown you get. And like most major one-way roads, the stoplights are timed to the speed limit. On any one of these roads, once I pass an intersection with a green light, I can set my car's cruise control to 30 mph and ride straight through to the end of the one-way road, right through all the stoplights, unless the road is heavily congested.

Contrast this with the two-way, two-lane roads that go through the busier part of town, which is where the one-way roads are. There is no way to sync stoplights for two-way roads, so where there are a lot of them, you have to stop frequently. At other intersections, there are stop signs. And even for major two-way roads without stop signs, there is generally one lane in each direction (as in SC4), and traffic turning both on and off the road tends to slow the road down considerably. For example, traffic on Willamette, which is one of the main roads in Eugene, tends to travel much slower than traffic on the one-way roads I mentioned, even over those sections where there are no stoplights. Overall, it's easy to see in this town how the two-thirds speed and capacity rule applies.

What about big cities? An excellent example is the biggest city in the U.S., New York - specifically Manhattan. There's a saying in New York something to the effect that, "You can go uptown, and you can go downtown, but you can't go crosstown." At first, this may seem a little strange, as Manhattan is a narrow island, and crosstown is the shortest trip you can make. But Manhattan has many big, wide one-way avenues going uptown and downtown, while the major crosstown avenues are two-way, even though they have the same number of lanes as the one-way roads. It is not unusual for the wide one-way avenues to carry traffic at several times the speed of the two-way avenues (technically they're called streets), even though the nominal speed limits are the same. And the East Side avenues are efficient enough at carrying traffic that unless you're traveling more than a few dozen blocks, it's really no faster to go over a few blocks to FDR Drive, a major highway that parallels the avenues.

Some may say that one-way NWMs shouldn't have a significant speed or capacity advantage over avenues. But in the real world, they do. Look at Manhattan again. On the Upper West Side, Broadway, which is a four-lane, two-way avenue, parallels Amsterdam and Columbus, which are wide one-way avenues. Anyone who wants to travel more than a few blocks by car will take Amsterdam or Columbus, as they are much faster than Broadway, even though the speed limit is the same. So for all these reasons, I don't think that raising the speed and capacity of one-way roads by a realistic amount makes a traffic simulator not compatible with the NWM. To the contrary, I think such a change makes it *more* compatible.

What about all the side streets in Manhattan that are one-way? They don't have the advantage of timed lights, and their speed and capacity is about the same as normal streets. Recently, a proposal has been made for one-way streets in SC4. I think this is great idea, as they would have the same capacity and speed as regular streets, and would provide a nice alternative to higher-capacity one-way roads.

Speaking of alternatives, boosting the speed and capacity of one-way roads would have yet another benefit. A long-standing complaint in SC4 is that all roads and avenues have the same speed limit. If one-way roads have a higher, more realistic speed limit, then players can immediately construct their own higher-speed avenues. Eventually, higher-speed forms of NWM could be made this way. I believe that higher-speed two-way roads could also be made using the one-way network; someone with more knowledge of this process can confirm or deny this. Assuming they could, they would serve as an excellent intermediate between standards roads and RHW-2. And unlike RHW-2, residences and businesses could be built right up against them.

I mentioned earlier that one-way roads are considered to give a two-thirds boost to capacity and speed, and I also mentioned that the current setup for Simulators A, B, and Z gives a boost of from 0% to 20%. I am proposing to increase the speed and capacity of one-way roads to be 50% higher than that of regular roads, which should make them come out about right. As I mentioned, this will immediately make faster hand-made avenues possible, while leaving regular avenues unchanged. What do people think about this proposal? If there is enough popular response, I can post a full release of Simulator Z with these changes in this thread within the next couple of days.