Yes, you are legally required to signal BEFORE merging. Merging onto a highway is technically the same manoeuvre as a lane change (you’re just driving faster). Signals are important in this case to warn traffic that you’re planning a movement, especially during those instances where the onramp and the offramp is the same lane.
Many moons ago, Carmen became an ICBC-approved driving instructor at the age of 22 in North Vancouver, and has spent many years working with new and experienced drivers around the lower mainland. She can be found reading the Motor Vehicle Act for fun while receiving strange looks from others. May the quest for great driving continue!