After observing people maneuvering motorcycles, scooters, and bicycles, I believe that I have discovered the over-riding principle. If one is able to move forward without tipping over, then the load is acceptable. Fragility and/or danger are not considered. These people are talented (or lucky), brave (or foolhardy), and ingenious (or desperate).
Automobiles have seats and seatbelts, which usually determines the number of passengers. The number of motorbike passengers is left up to the ability of the driver and fearlessness of passengers (not something I noticed much). I missed a photo of a bike with five passengers: three adults and two children. They could have been a circus act in the western world, but seemed rather nonchalant in the midst of raging Saigon traffic. I managed to capture only foursomes.
Although helmets were required, many seemed to flaunt the law and ignore the risk. Parents often set a bad example for their children.
Driving a car is dangerous enough, but talking on a phone on a motorbike includes even greater risks. I will not even comment on texting and biking...
The first time I spotted someone transporting plate glass on a motorbike, I assumed that this was a foolish practice; the second time, I decided that it was common.
The inventor surely did not envision a motorbike being used as a truck...
And, finally, the mobile home version...