Procedural City Stop Signs

Up to this point, 3DWorld's procedural cities have traffic lights at every 3-way and 4-way intersection for both roads inside the city and connector roads between cities. This works well for cities that are full of cars, but it may be overkill for residential neighborhoods with lighter traffic. Stop signs may be a better fit for these situations. I've added stop signs to residential cities, except for the intersections with roads connecting to other cities, which continue to use traffic lights.

It was easy to find a stop sign texture to use. Each sign is a two sided textured quad with the red "STOP" texture on one side and a white octagon on the other. The sign is placed on a square post, with an additional smaller "4-WAY" sign underneath the main sign at 4-way intersections. These are placed at all corners facing cars that will be entering the intersection. Stop signs are positioned close to where the traffic lights would be and can mostly use the same pedestrian and player collision detection system.

The next problem was adding street signs. I really like how every road has a unique procedurally generated name, but there's no traffic light to attach the signs to. It doesn't look right adding the street sign to the top of the stop sign with a vertical extension. So instead I added a second, taller vertical pole just behind the stop sign with the street name on a green horizontal banner at the top. I had to make this high enough that the largest vehicle (the truck) can pass underneath.

Here is an example stop sign on a 4-way intersection with cars and pedestrians enabled.

A 4-way stop sign with a street sign above and behind it. Cars and people are both waiting to cross.

 

The most difficult step was writing the traffic logic that controls cars reaching stop signs. As with traffic lights, the only real function of this stop sign logic is to tell each car that's about to enter the intersection whether it must stop or can proceed. Once the car is in the intersection, it's free of the intersection control logic and only subject to the car and pedestrian collision checks.

The first step is obvious: Each vehicle must stop at the stop sign, independent of the intersection state. This is accomplished by braking/decelerating when approaching the intersection, starting a timer when fully stopped, and waiting until 0.25s has passed before querying the intersection for permission to enter. This small time delay is enough to prevent a rolling stop. I may decide to make the wait randomly generated per-vehicle to add some variation.

The next step is to determine whose turn it is. We conveniently already have a timer recording when each car reached the intersection for stopping purposes. Each car will then check if there are any other waiting cars with earlier times that should enter first. This check only applies to cars that will cross paths, using the following logic:

• If cars start at the same location, this is an error.
• If cars end at the same destination (one of the 3 or 4 exit directions), they definitely cross.
• If either car is turning right, they don't cross.
• If either car is turning left, they cross.
• If we got here, both cars are going straight. They cross if they're going in different axes (one north-south and the other east-west).

I later discovered that the trucks make wider right turns and will in some cases clip through other vehicles making left turns. So I had to add an extra step into the logic:

• If either one is a truck, one is turning right, the other is turning left, and their paths are diagonally opposite each other, then they cross.

This set of cases appears to be correct. But it's actually more complex than this, because we also have to consider other cars currently in the intersection. My solution was to have cars register themselves with the intersection when entering and remove themselves when exiting. The intersection has 3 or 4 slots corresponding to its entry points that store the turn direction and is_truck flag for each car that's using it. Then the intersection must run the same logic above on any active slots that cross the path of the current car that is asking for permission to enter.

The next problem I ran into was a deadlock case where cars were waiting at each stop sign but no one was going. This took me a long time to reproduce, debug, and fix. I eventually had to add code to detect this case by looking for cars waiting for more than 60s, show where it was happening in the world, and print debug state for the intersection. It turned out that this was caused by cars blocking the intersection because there was no space for them to turn and exit the intersection. This was because cars were queuing up waiting at a traffic light to exit the city, and the line of cars was backing up all the way to the intersection. This in turn was causing the car attempting to enter the intersection to stop while partially inside. The problematic car had already set the "entering intersection" flag for that lane/direction as it had started to move. But when transitioning back to a "waiting" state while space for it to turn became available, it was actually blocking itself from continuing when it was able to. The fix was to have a car clear it's own intersection usage state when stopping while inside and intersection. This will also allow other cars to continue while one car is blocked, which may or may not be correct. I think this is okay because we know they can't pass the initial car as the destination spot is known to be blocked by another car. Therefore, they must be waiting as well, and only one car can claim any spot that opens up.

This overall solution mostly works. There are still some rare cases when cars spawn inside an intersection, or a lag in frame rate causes them to cross through an intersection without properly registering a matching enter + exit event. If left unchecked, this can lead to deadlock where none of the cars can enter. I think one correct fix is to record time stamps for entry events and automatically remove them after a few seconds in the per-frame logic update pass of each intersection. However, I haven't seen this happen after some other tweaks, so it's either fixed now or very rare. I may have to let the simulation run for hours in the background to see if anything fails with the stop sign logic.

Here is a video showing cars following stop sign rules at an intersection. The only issue is that trucks cal still get too close to cars when turning. This can lead to odd behavior as they attempt to avoid collisions that shouldn't actually be possible. I haven't come up with a good fix for this, so I'm calling it done for now.