Sometimes, when the traffic was a gnarling mass, they would fly over the city in the diagram/sculpture which they kept in their pocketses.

(They knew all moments carried the same Weight - apart from a few exceptions - and so used up their diagrams, which they never hesitated

To haul from their pockets - along with 

Protractors, compasses, rulers etc.. for 

Reworking or tweaking, to suit the flow, and to generally avoid the gnarl).

S