Passengers
From Cantr II Wiki
A passenger is any person within a vehicle, be it road, sea, or air. In Cantarian context, a passenger always classifies the driver of the vehicle as well. Therefore, a vehicle holding 1 passenger, can not hold two people, only one - the driver.
Also note, a passenger weighs 60000 grams, inside and outside of a vehicle. Take this in mind when entering vehicles and buildings. If the vehicle or building can not support an additional 60000 grams, you will not fit!