When you must park on the roadway, which lights does the manual tell you to turn on?

The manual names the four-way flashers, not the parking lights, for a car stopped on the roadway. Parking lights are dim marker lights meant for a legally parked car in low light, not a warning to traffic that a vehicle is blocking an active lane. This answer matches the parking rule to the dashboard control that shares its name, on the assumption that lights called parking lights are what any parking situation calls for.

The manual names the four-way flashers, not a turn signal, for a car parked on the roadway. A blinking turn signal reads as an intent to turn, not as a stationary vehicle. This answer borrows the pulled-over-shoulder habit of leaving a turn signal on as an improvised hazard marker.

The manual names the four-way flashers as the signal here, not the headlights. High beams point down the road and dazzle oncoming drivers rather than mark a stopped car. This answer borrows the driving habit of flicking high beams to warn another motorist of a hazard, treating them as a general-purpose alert signal.