A ferry crew directing cars into specific lanes is executing a stowage plan, not managing a queue. The order vehicles go aboard determines stability, deck clearance and how quickly the ship unloads.
The ship must stay in trim
Vehicles are heavy and mobile weight, so loading them all to one side or one end changes the vessel's list and trim measurably before it has even left the berth.
Crew therefore load in a pattern that balances the deck, usually working outward from the centerline and forward and aft in alternation.
Heavy commercial vehicles are placed low and near the centerline for the same reason, which is why freight is loaded before passenger cars on most services.
Deck height sorts the traffic
A vehicle deck has a fixed clearance, and taller vehicles can only use decks or lanes that accommodate them. On many ships that means the main deck only.
Hoistable decks let operators reconfigure the space between sailings, raising or lowering a mezzanine according to the mix of cars and trucks booked.
This is why booking asks for vehicle height and length rather than just a headcount. The measurements feed directly into the stowage plan for that sailing.
Discharge order is planned before loading
On a route with more than one stop, vehicles for the first port are loaded last so they come off first. Nothing can be moved past another vehicle once the deck is full.
Even on a single-destination run, the crew plan for a clean sweep off the ramp, since a vehicle that will not start blocks a lane and delays the whole discharge.
Drivers are asked to leave vehicles in gear or in park with the handbrake on, because a rolling vehicle underway damages everything around it.
Lashing and access rules follow from the plan
Vehicles carrying certain cargo, or motorcycles that cannot stand unaided, are lashed to deck fittings, which requires space around them that the plan has to allow.
Passengers are generally required to leave the vehicle deck for the crossing, since it is an enclosed space with vehicle fluids, poor ventilation and no permanent lighting for pedestrians.
Returning to a car early to collect something is usually refused for the same reason, so anything needed during the crossing has to come out at loading.
What this means for arrival time
Arriving early does not buy an earlier place off the ship, because position is assigned by the plan rather than by queue order. It does buy a place on the sailing.
Arriving late risks being loaded last into whatever gap remains, or being rolled to a later departure if the stowage plan is already closed.
On a road trip built around a ferry leg, the sensible margin is set by the check-in deadline rather than the departure time, since those can differ by an hour or more.