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Contributor/author(s): S. A. De Caro, Jack

Interchange systems enable motorists to merge onto orthogonal highway routes, with little or no disruption to the flow of traffic. In SimCity one can design an effective interchange system that is free of overlapping highway segments. The default overlapping technique in SimCity 3000 uses an overpass segment and four elevated ramps. For some, this approach creates an ugly depression and bump along the two divergent highways, but it also requires a mixture of road-to-highway and highway-to-highway ramps to be 100% effective; and it is not designed for terminating highways. This article addresses the construction and implications of a T-junction interchange (where one highway terminates perpendicular to another), but the design is easily extended to support complete throughway of traffic on non-terminating highways.

Phase 1: The simple T-junction

Figure 1 shows the construction of a simple T-junction. For reference, it is supposed that the major highway is running north-south, with north oriented at the upper left corner. I'll refer to this highway as H1 (and the particular routes as H1-N and H1-S). The second highway will be designated as H2 (and the particular routes as H2-E and H2-W). To physically connect the two highways, position the blue construction zone (a 4x4 highway segment) so that it overlaps H1 and H2; then shift one tile to the right, as shown in Figure 1. Repeat this process for the single open tile to the left of H2.

Another option is to build a full interchange type T-junction, as seen below. This may be more desireable since the above intersection may not allow cars to pass on all highway tiles due to the fenced highway tiles getting in the way:

Sims can't get to work

The effect of the interchange system being developed here was tested by restricting all residential zones at the north end of H1 and all industrial/commercial zones at the south end of H1, thus making the H1-S route critical to city development. In this scenario, the simple T-junction does not disturb H1-N traffic, and it has opened an immediate connection to H2-W traffic; but only for motorists already traveling H1-S. Indeed, two traffic routes are necessarily diverted by the physical connection of H1 and H2: The critical H1-S traffic is diverted onto H2-W and H2-E traffic is diverted onto H1-S. But Sims are reasonably determined about getting to work. It turns out they will try to drive the full length of H2-W, turn around, drive back on H2-E and then resume the H1-S route to work. The problem for mayors is that this interchange system is inefficient. The Sims know it, the Transportation Advisor knows it, and you'll know it too when the 'Sims Can't Get to Work' message scrolls across the screen. The frequency and duration of this type of traffic problem is at least partly contingent on the length of H2 (i.e., the distance by which traffic is diverted). For instance, if the length of H2 is relatively short, the simple T-junction shown in Figure 1 will suffice as a complete interchange system and will fully support 'No Traffic Problems'--with perhaps only intermittent incidents that are brief enough to be ignored. But if your highway system is more extensive, designed to connect disparate regions, then it will demand a more elaborate interchange.

Phase 2: Full interchange system

 

Regardless of the frequency and duration of traffic problems resulting from diverted highway traffic, the interchange system represented in Figures 2a and 2b is my preferred remedy. This particular Level-1 system requires a modest 18 road segments, plus the six on/off ramps. The strategic use of internal overpasses minimizes the amount of land required for the system and it promotes a more realistic interchange system, similar to the systems used routinely in downtown areas.

Note, however, that this particular version is highly simplified. While it is 100% functional (and suitable where space is at a premium), its compact structure will generate ramp congestion even in modest traffic. It is the job of the mayor to determine how many road segments are required to avoid ramp congestion. For example, Figure 3 shows a Level-4 interchange system. In this particular scenario, moving all ramps three tiles was sufficient to maintain an acceptable level of ramp congestion.

Test results

If one monitors the traffic animation carefully, it appears as though the majority of motorists are not using the full interchange system properly (e.g., many Sims may continue driving westbound to pick up H1-S). However, the efficacy of the full interchange system is confirmed by its effect on traffic problems and city development. Exclusive use of the simple T-junction shown in Figure 1 was concomitant with regular displays of the 'Sims Can't Get to Work' message. [Identical traffic problems arose when the default overlapping system was implemented in a similarly constrained scenario.] These traffic problems were eliminated by implementing the full interchange system depicted in Figure 3. Collapsing the full interchange system back to a simple T-junction restored the original traffic problems. Hence, Sims really do prefer a full interchange system for their highways, and they will make efficient use of it.

See also
Beautify your highway system
Highways, for aesthetics only?
Highway interchange: cloverleaf
Y-Splits for highways
Yet more cloverleafs
Highway traffic-circle interchange
Highway interchange: yet another
A more aesthetic highway interchange system
How to make a highway tunnel
Transportation infrastructure
Building the perfect mass transit system

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