When passing cyclists, when is it mandatory to occupy the entire adjacent lane, changing lanes completely?

Correct answer: When the roadway has more than one lane in each direction.

When the roadway has more than one lane in your direction, you must move completely into the adjacent lane to pass cyclists, like the passenger car in the image. With only one lane in each direction, a complete lane change is not required, and you use as much of the oncoming lane as you need to leave at least 1.5 meters of clearance.

Real Decreto 1428/2003, de 21 de noviembre, por el que se aprueba el Reglamento General de Circulación · Real Decreto Legislativo 6/2015, de 30 de octubre, por el que se aprueba el texto refundido de la Ley sobre Tráfico, Circulación de Vehículos a Motor y Seguridad Vial