I started a new world in Sunset Valley with Overwatch and Awesomemod active. By default Overwatch cleans up stuck vehicles in the world at 3:00 AM. For the first 50 days or so in my game the number of vehicles was no more than 20. Suddenly, the next day during cleanup Overwatch reports ~1000 or so stuck vehicles! And the number seems to be increasing almost exponentially...the next cleanup reported about 2000 vehicles.

Any idea what might be causing it? I've installed ellacharmed's mod of Sunset Valley to repair broken routing paths so there shouldn't be THAT many errors.

Thanks!

This happens as you play. The longer you play the more vehicles you need to clean up. It fixes itself when you log out and log in again. I type "towderelicts" a couple of times a day when this starts happening and Awesome Mod gets rid of them. I don't know what causes it. When I log out of sims and come back in it doesn't do it again until I have played for hours.

Oh, I didn't know about the TowDerelicts command. My game runs way smoother now, thanks!
I did try quitting the game every now and then but the number would still increase every time I reloaded that save.

I use the NRaas Traffic mod to disallow inactives from being issued temporary cheap cars to get around that later get abandoned, they have to take taxis instead if they don't have their own vehicle or it's not available. Once we start talking about vehicle cleanup in the thousands though, it's more likely due to performance career limos or snack/ice cream truck spawnage getting stuck in a loop as mentioned. Traffic has settings to manage those better as well.

Routine cleanup is great, but I feel better about these things generally if they are not overspawned in the first place.

I think that AwesomeMod has a new (from a few months ago) routine that deletes NPC cars used by inactives directly after their use. I use Overwatch in conjunction with AM and for quite some time now I haven't had more than a couple of cars deleted by Overwatch each night if any at all (0 cars is rather the rule than the exception).
But:
There is a bug having to do with the multiplication of limousines used by various careers and special occasions and the bus for Sims in the rockstar career. They multiply in the background by the hundreds and do not even appear visibly in the world. And if you change households a lot during a game session the problem will even get worse.
For this to stop you will need a third mod that prevents these vehicles from spawning, I guess either NRaas Traffic will do or nonamena's No Limos. However, this mod will not address the limo issue with some careers (Fortune Teller, for example) because they are managed by different files. The link I posted will also give you her second mod "No Limos Careers Plus", but you should read up there why this mod might cause further conflicts.
Also: neither of nonamena's mods will address the issue with the music career buses but at least this can be fixed by editing nona's original mod manually for these.

While Nona's mod may conflict with some, there is no problem using her No Limos Careers Plus or any of her other flavors together with NRaas Traffic.

I guess that if you use Traffic you won't need nona's mod anyway, will you?

Traffic.
One more mod to add to my NRaas.package. Thanks.

On the limos, Traffic only addresses the overspawnage of Showtime performance career limos. While that's the most common cause of the issue, with the snack/ice cream trucks being second, Nona's No Limos Careers Plus version takes care of the other kinds. I use both mods myself as do many others. After a while, many of us just found ourselves saying, "For heaven's sake, grab a bicycle (or broomstick) or take the subway like everyone else!"

Thanks for the suggestions everyone! I downloaded both Nraas traffic and Nona's mod for good measure. Overwatch rarely detects any more stuck vehicles and my game runs smoother too.