The road ahead narrows and the painted lines angle across to guide you over. Should you expect a warning sign as well, or do the lines do the whole job?

Correct answer: A. Expect a warning sign too — transition lines may only be used to supplement standard warning signs, never on their own

A sign as well, always. Book 11 treats a narrowing road as a trap: "any reduction in pavement width is an expectancy violation and a hazard to the driver, especially at night", and then states the rule outright — "a transition line alone is not sufficient warning of a reduction in pavement width. Pavement width transition lines must only be used to supplement standard warning signs." Two other things happen in a transition area worth recognising from the seat. The opposing directions "must be separated by a barrier line in the direction of convergence", so passing is shut down through the merge. And the painted lines "may be supplemented by arrow markings on the pavement, placed before the point at which convergence begins, directing motorists to merge from the discontinued lane into the through lane" — those arrows appear before the squeeze, not in it, which is your cue to move over early.

Source: MTO Driver's Handbook: Ontario Traffic Manual, Book 11 — Pavement, Hazard and Delineation Markings (March 2000), Section 3.5 — Transition Lines and Continuity Lines