`POST /v1/route/heritage` (see module docs).

The route's own duration in seconds, typically duration_s from the same response. Supply it: without it the silence budget falls back to a distance, and "three to five lines an hour" is a claim about time that a distance answers wrongly at both ends.

The route shape as a Google encoded polyline, six-digit precision, typically geometry_polyline6 straight out of a prior /route response.

Ceiling on how many lines come back (1 to 50, default 12).

How near a plaque's own recorded coordinate must come to the route before its line may be spoken, metres (default 60, hard ceiling 80). Past about eighty metres the subject stops being something the driver is passing, so the ceiling cannot be lifted from a request body.

Least distance between two spoken lines, metres (default 400). A floor under the time budget in every case, and the whole budget when no duration is supplied.

Least time between two spoken lines, seconds (default 900, which is four an hour). Applied only when duration_s is given.

Container to give each line's audio clip address in: opus (Android) or m4a (iOS). Given, every line gains a clip with the synthesis cache key and the URL to fetch it from. Omitted, no clip is computed and the answer is text only.