[jrootabega]

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.

[taurknaut]

How is clumping indicative of bad driving?

[jrootabega]

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...

[therein]

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.

[greentxt]

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.

[greentxt]

Left lanes are for passing. If you are in the left lane and not passing, guess what.

[jrootabega]

I agree that clumps usually end up blocking passing lanes, but they're also bad when in any lane(s).

[greentxt]

A clump == blocking the passing lane. They are definitionally the same.

[taurknaut]

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.

[pglevy]

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.

[tristor]

Because it indicates that they are not driving to the conditions of the road and being attentive.