The Grande Raccordo Anulare (GRA) is Italy’s most expensive public highway; a 50-km ring road that encircles the city of Rome. If you listen closely to documentarian Gianfranco Rosi’s freewheeling chronicle of this outsized bypass, you can practically hear the ghostly flutter of brown envelopes under the hum of traffic.

An argumentative father and daughter (who, we learn, once lived in a more salubrious location up north), stuck in an apartment in the outer reaches of the city with planes taking off overhead, give us some sense of place: that is, cheap, small and extremely satellite. Mostly, however, Sacro Gra – named as slant rhyme for Sacro Graal (Holy Grail) – has no fixed abode.

Inspired by Italo Calvino's Invisible Cities, a postmodern reworking of the journeys of Marco Polo, Rosi adopts hypno-doc pacing as we hop randomly from character to character. There are plenty of eccentrics in the cast. By the roadside, a tree doctor hunts weevil larvae in palm trees. A dismayed fisher of eels reads aloud from an article about foreign eel invaders in the River Tiber: "This is real crap, Irene," he tells his wife. A paramedic skypes unspecified relatives between patients. A crowd stares into the sun hoping to see the Virgin Mary.

There are moments of pathos and moments of mirth, but Sacro Gra stubbornly refuses to coalesce into a proper film. A gentle, observational piece, shot mostly from static cameras, there is little evidence an organising principle, even one derived from Calvino's avant-garde styling.

Rosi’s latest documentary was the surprise winner of the Golden Lion prize at last year’s Venice Film Festival. Surprise in this instance means everyone scratched their heads and wondered what on earth the jury were up to.