OpenLoco
Original game bug: AI companies build roads with lots of unnecessary turns
I wrote up a big wall of text about this on the Gitter, so I guess I'll save some time and replicate that here:
The plane left/right/left/right thing is very similar to how the idiot AI's build their ground vehicle routes. Particularly roads. Suppose they are building a truck route and need to do an overall diagonal path to get from point A to point B. Well, roads in this game, unlike rails, can only face 90 degree directions: there's no 45-degree half-turn piece. The only turn options you have for roads are a single-tile tight turn and a 2x2-tile wide turn. Given that these constraints are in place, the optimal thing to do would be to build the road so that it primarily consists of one long stretch at one 90 degree angle, and then another long stretch at the other 90 degree angle (so that the hypotenuse of the triangle goes directly from point A to point B).
But the AI wants to make a diagonal road really, really, really badly, and so it will build its road as an approximation of a diagonal road, by alternating left and right turn pieces back and forth. But of course, as you cannot build a true diagonal road in the game, there's no advantage to making a crude approximation of a diagonal road out of a bunch of 90-degree turns; it's still effectively equivalent to building an L-shaped road with two long straight 90-degree segments. (I guess technically the 2x2-tile wide turns would get a very slight distance reduction compared to the L-shape, thanks to Pythagoras.)
Now, the consequences of the AI's decision to make a futile attempt at building a diagonal road using back-and-forth turns are that (a) it looks ugly and stupid, and (b) the trucks travelling on the route have to slow down on turns, especially the tight turns; there are max speed limits for each different turn radius. So particularly in more modern years when the available trucks have a higher max speed and could take advantage of it on long straightaways, the AI's zigzag roads ensure that their trucks will constantly be travelling well below their maximum speed. Meaning that making the road "diagonalish" ends up being completely counterproductive for them.
It remains to be seen whether the AI's road construction logic consists of trying to stay as close to a straight line from point A to point B as possible; or if perhaps it does some sort of optimization for shortest path distance, and the 2x2 turns have a distance value assigned to them which leads the AI to believe that a path with more turns will be shorter overall (due to 2x2 circle circumference versus 2x2 square perimeter: 2*pi is around 75% of 8); or if there's some other idiocy going on.
The ultimate fix here is to make the AI realize that building a road with an excessive number of turns, especially 1-tile tight turns, involves a substantial speed penalty for their trucks/buses, making the turn-heavy route actually inferior to a straightaway-heavy route in the end; and have them factor that into their decision of the route they decide to build. (At least for the cases where the vehicles that the AI intends to put on the road have top speeds sufficiently high that the turn-radius-enforced speed limits would force the vehicles to slow down considerably. For some of the earlier-year vehicles, it doesn't matter as much, since they have relatively low top speeds anyway.)
Perhaps the AI logic could do a time-to-traverse-the-route simulation when it's deciding which hypothetical road to build (factoring in the vehicles it plans to use and the speed limits at each piece of road it will follow due to turns and bridges and so forth), so that it can actually compare between a straightaway-favoring route and a turn-favoring route and have the matter of "oh wait, these turns mean that it will actually take longer to get there, despite the route ostensibly being closer to an optimal diagonal path" come into play.
Or, a simpler approximate way to implement this might be to just arbitrarily assign turn pieces a "negative point value", if you will, that makes the AI try to avoid using them unless they are truly necessary to make building the route possible or to avoid making an expensive alternative route that involves bridges or whatever. 2x2 wide turns get a mild negative selection bias, and 1x1 tight turns get a much harsher negative selection bias. Something like that.
