First of all I have to say that this is partly from what I've found out myself and partly what I learned online, so I certainly can't take all the credit for it. Similar solutions I saw went too far and deleted or changed far more than was necessary. And also that I'm still sort of guessing about a lot of it, so don't take it as gospel. I'm no expert. But it worked for me.

When I first installed The Sims 4 and started playing, it would go for many hours without crashing, then it began crashing randomly more and more often, eventually crashing every 20 min or so. After the fix, I've now played for several hours without a crash, including travelling.

I found the problem to be one or both of the following; Duplicate sims, or sims created with custom content (cc) that is no longer available, that exist in your 'my library' and/or in 'household management'. Sims created with cc are sims that needed mods or expansions that were removed/uninstalled. For example, say you gave a sim a shirt from a stuff pack and then uninstalled/reinstalled without that stuff pack. That sim will be broken and when it tries to spawn it will crash the game.

I don't know why (probably my own fault, and if I did it, I'm sure others have too), but when I first created a sim, and saved it to 'My library' I later found out that I had 2 copies of my sim there, both created at the same time on the same day. And when I looked in 'household management' there was a duplicate there as well. Somewhat later, I had also made 2 sims using cc, and later removed the cc, and didn't delete the sims themselves.

Anyway...

To fix, definitely do this first part. I'm 99% sure this is the problem.

Enter 'household management' (upper right) from 'manage worlds'.

Select 'unplayed households' then select 'not in world' and delete any duplicates of yourself and any sims that may have been made with cc that is no longer available. I think that will fix most crashes.

You may need to do this part as well. Not so sure about this though.

Go to 'My library', and on the left select 'advanced' and select 'include custom content' then select 'view content by' and 'mine' then select 'households' and delete duplicates of yourself, and any sims that may have been made with cc that is no longer available.

And if you have downloaded another sim from the gallery, that needed cc that you no longer have, you should remove those as well, by selecting 'view content by' and 'others'. Same for the 'household management' section. Though that's harder to determine who is who.

The problem seems to be that it's trying to spawn either your duplicate or a sim with expired cc into a lot and that crashes the game. And as it populates the sim world there are more and more chances of it choosing one of the bad ones.

I think it may be able to do the duplicate one without crashing, since I've heard of people having that happen, but I'm not sure.

The reason it crashes for so many people when they travel is that when you travel, it randomly spawns a bunch of people and that increases the chances of a crash, and at home it spawns people too, but not so many at once, so it will crash less often.

Save often anyway, just to be safe.

Of course, your game may be crashing for a totally different reason, but at least check and see if any of the above stuff is going on.