I recently travelled on BC Ferries (in British Columbia, Canada) and I made an online reservation for my car and me, which is billed 15 dollars per trip. When arriving near the ferry terminal through one of the main highways, there were traffic screens giving real-time filling of the ferries. All of the indicated ferries were rarely more full than 50%, some were less than 10% full a few hours before departure. This means few people reserve their ferry spot beforehand. I supposed that showing up early was enough to have a spot, at this time of the year at least (end of June).
Since 15 dollars per trip is a bit expensive (the whole trip is less than 100 dollars), I would like to know if there is a way to predict when the reservation is useful, i.e. when showing up 30 minutes early is not enough to have a spot on the ferry (and not wait for the next), if it happens at all that ferries are full? I am mostly interested in the ferries between the continent and Vancouver Island, but ideally it may apply to all BC Ferries.