I noticed a similar phenomenon while driving on the highway. Vehicles on the other side of the road seem to group up and travel in clusters. Like traffic naturally oscillates into these packets?
The traffic waves mentioned in the other replies are not the same as clumping. People clump up while driving, even on the emptiest of highways, because they are bad drivers. This was happening long before automatic braking, but that has made it more common.
Unnecessary clumps are bad. They make merging and passing harder, desensitize you to nearby cars' presence, cause night vision problems, etc.
A good driver will first notice they are in an unnecessary clump, and a bad driver probably won't.
A good driver will try to minimize the the time they spend relatively motionless to other cars (other than behind them at a safe follow distance). A bad driver either doesn't realize as in the previous point, doesn't care, or actually prefers it as some kind of crutch. Sometimes the leader of the clump ends up that way because they will speed up whenever those behind them try to pass. Sometimes the rear of the pack is a driving zombie who will close any distance between them and the car in front, but will never choose to pass. And so on...
In the Bay area, what I noticed is, unnecessary clumps almost always get nucleated by a Tesla going lower than the speed limit on the left lane and then a tall vehicle behind them.
The tall vehicle behind the Tesla will hide the fact that the slow Tesla on the left lane is slow for no reason, people behind the tall vehicle will think there is traffic. And we will all go 55 on the left lane, thinking there is traffic. Until some incline allows people to see that there is no real traffic and then the Tesla gets overtaken and clump is broken.
Stay in the far right lane unless passing. Don't tailgate. Those 2 rules would solve a lot of problems if they were enforced. Instead, there is heavy enforcement of the speed limit, which in and of itself isn't a safety concern.
I think about a large cluster of cars, say 6-12 cars, going roughly the same speed. Nothing about that entails blocking multiple lanes, certainly not densely enough to preclude someone from passing.
Guess what... Sometimes I need to take a left exit. And there will frequently be a bunch of clumping chumps doing 20 over in the left lane. So I get over when I can.
I had heard of traffic waves in congestion, but this phenomenon happens on open roads. I often notice vehicles forming packets when there isn't traffic. Like natural clustering? If I drive the speed limit, cars will start to bunch up behind me, even when there is an open lane. Then, a driver will speed up in the passing lane, but when they reach me, they often slow down slightly or a lot and ride next to me without passing.
There is very specific clumping behavior on the I5 in Northern California I've noticed. It is a long, 2 lane highway often used by truckers. It tends to be fairly congested but not ever backed up.
What I see every time I drive it is a new clumping behavior that emerges: some big rigs tend to drive quite slow but do that thing where they want to pass someone going 54 and they are going 55 so it takes ages and the traffic in the left lane gets longer and longer, leaving a gap in the right lane.
At thing point, some drivers say "fuck it" and zip down the right lane and aggressively merge back in to shorten their wait getting around the blockage. As you are likely driving this route for hours (SF to LA) you see it happen more and more often and it becomes more and more prevalent. I find it exhausting because there is no way to veg the whole drive unless you are the slowest car out there because at best someone is going to jab in front of you at the last second and you have to mash you brakes to avoid them.
The interesting thing is that this is always how the traffic flows on this road, and nowhere else. It must be a precise combination of traffic levels, lanes available, traffic mix, differring speeds and even average distance travelling, so people notice and feel compelled to respond to the pattern.
In my experience, anywhere there is heavy semi truck traffic will have significant problems once there are enough cars around ("enough" being far less than it should be). 2 lanes, 3 lanes, 4 lanes, hills, it's all bad. But yeah, only 2 lanes per direction makes it comically awful. Happens on sections of I-81 too.
I think the specific pattern you're talking about happens because when a truck is passing slowly but infrequently, it makes some sense to be polite and line up behind it. When trucks are passing so frequently, and so incredibly slowly, being polite is useless. There needs to be a zipper merge happening behind the truck being passed. But the left lane people feel they've paid their dues by waiting in line, and half the right lane people don't even care. The right lane people also stay so close to the truck in front that they can't safely merge left at highway speeds anyway. People also tend to not fill back in to the right lane between truck passes. I'm not sure that would even help, but it does make people who do seem like cheaters to the people who refuse to.