groups of stars become separated when passing through dense traffic

An example of metaphorical language in popular science writing:

"These large globular clusters move along orbits that lie for the most part quite outside the plane of the Milky Way. In astronomical phraseology, they spend most of their time in the halo of the Galaxy, not in the disc. And this is precisely why they have managed to survive for so long as separate compact groups – the stellar traffic along their orbits has been low. But similar groups in the disc of the Galaxy moved along orbits where the traffic density was high, with the consequence that they were slowly loosened and their constituent stars gradually escaped into the general field…

The recently formed smaller groups are disrupted still more easily. Indeed, some groups, such as one in the constellation of Perseus, seem to be disrupting spontaneously. Others separate only by passage through dense traffic. The famous group of the Pleiades is not one of the spontaneous disintegrators. But the stellar traffic will inexorably scatter the Pleiades, until in about 500 million years little will remain of this at present spectacular cluster."

Fred Hoyle (1960) The Nature of the Universe (Revised ed.), 1960

A number of terms here seem to be metaphorical in origin, especially the references to stellar 'traffic' ,