dabreegster
commented
4 years ago It's a bug, perhaps the most infamous one: gridlock. I've been fighting in some form since ~2011, when I was a lowly undergrad working on a different simulator. :)
It has many causes, but you've hit upon two of the big ones. For the traffic signals, there's an attempt at dealing with this, but it's incomplete. If you press ctrl+S in game to activate dev mode, then click one of these pairs of signals, there's a button to explore uber-turns. I'm trying to make the traffic signal editor understand movements across pairs of intersection that people think of as logically one intersection.
Long cars getting stuck in short roads has a bunch of causes. I have some hacks that I've hardcoded for some intersections that sort of help; they may help with Main and 1st, but may not.
Anyway, this is a hard problem always on my radar. I'm working on a comprehensive presentation of the modelling assumptions in A/B Street, which will definitely cover this topic. I'll link that here when it's ready.
belt-drive
to
matkoniecz
Another problem came up in the SLU map due to one of these, prompting discussion again about merging the short "road" between these two "intersections" into one.
Decent in some cases, covers way too much area in others. This is a problem especially when the merged shape cuts across an incoming road at a non-90 degree angle.
Trying this out again at the 
From a few test cases, fixing the intersection geometry wasn't hard at all. :D Testing a fix more rigorously, then blazing forward. No short roads shall survive into 2021.
Otherwise we'll wind up with U-turns, depending on the (arbitrary/unpredictable) order that these junction ways get merged:
Not sure if this is a feature or a bug, but reporting here because the behavior seems unexpected, or at least undesired. Working in the sandbox with the South Seattle map.
Roads with medians, like 1st Ave S in downtown Seattle, can cause permanent backups. These are roads that are treated as two separate one-way streets that run parallel. These backups seem to result from the very closely-spaced intersections that occur with cross streets where there is only space for one car in the space between the intersections with the respective sides of the street.
Seems like there are two primary reasons (possibly more) why these persistent, unresolvable backups occur at these locations: 1) If the signals on the two sides of the road, i.e. the two one-way streets, are out of sync, there is capacity for only one car to move through the intersection per light cycle because of the one-car holding zone in the median intersection. Oh well, poor light timing is maybe one of the challenges of the game that the player needs to fix, so not necessarily a bug here. One car per cycle is better than no cars per cycle. 2) This seems to be a bigger issue, though: a long car can stick out from the median holding area to block a tiny portion of the main intersection, effectively shutting down that intersection. If one car sticks out from the median holding area after every light cycle, the intersection is effectively closed permanently. Seems like cars will never try to pass through an intersection if any portion of another car from a different light cycle is still in the intersection.
See attached screenshot. There is a car turning left from South Main St to go north on 1st Ave S. This car is in the small "holding area" in the median of 1st Ave S. The tail of this car sticks out a bit into the intersection of 1st Ave S heading south and S Main St. The two cars that are trying to go south on 1st Ave S through this intersection have been waiting more than an hour because there is a always a small section of a car sticking out into the intersection at the end of every light cycle, preventing them from going forwards. They're stuck forever, and that permanent backup eventually ripples out through the rest of the street grid.
So, maybe this is a feature and not a bug, but it's annoying. Likely also a known issue. A few possible ways to fix it: 1) From the dev standpoint, allow cars to go around partial blockages to traverse a partially-blocked intersection. That sounds like a complex can of worms, though. If you can solve that problem, you're most of the way to being the first AV billionaire. 2) Somehow enforce that every interstitial intersection space can accommodate at least one car. Not very elegant or in keeping with the idea of a realistic streetscape. Plus, big cars causing problems when trying to use small streets is a real issue on streets like 1st Ave S in Seattle, though I think most of the median spaces on 1st Ave S and other streets with similar configurations (Beacon Ave S, for example) can accommodate at least one car. 3) As a user, close all those tiny problematic between-intersection spaces. Close for construction or turn them into the world's most useless bike or bus lanes; anything to keep cars from trying to use them.