What I call a cluster bomb is a phalanx of cars, most often on the interstate, that form a close pack some four or five cars wide and three or four deep.
There is no standard name for this phenomenon: cars grouping a length or less apart when the traffic would permit more distance. It's like a school of fish, a flock of birds, a swarm of bees. It hangs together.
I avoid cluster bombs, let the whole roaring mass flow around me. But perhaps some drivers enjoy the proximity of other cars, the hivelike sound of cars to the left, cars to the right, cars ahead and behind. Maybe it feels like you're part of a team. The sound of speed nearby is stimulating, like the sound of a locomotive or jet engine, and your right foot gets heavy in response. Let's go, go, all of us go.
To increase speed is to say Yes.
You have to drive at cluster speed, so perhaps it confers group security against being pulled over. It allows a sort of group bullying, too--sometimes it's hard to avoid being flanked by cars and swept along.
Interstate 270 south of US 40 takes a long steep downhill slide to the valley of I-44. Interstate 44 itself has a particularly pitched pass near Six Flags (enough said), and Interstate 55 south is like a roller coaster between the Gravois and Gasconade exits.
A cluster bomb going downhill is a metal landslide, five lanes and many tons of momentum. The stuff that pileups are made of.
Driving less than a car's length from other cars seems patently lethal to me, which is why I keep my distance. Is it possible some drivers feel safer?