Boarding a train or bus, passengers instinctively look for a seat with an empty one next to it.
Air travellers long for a spare seat beside them, and in the old days, hit the jackpot on long-haul flights if they were lucky enough to find they had a row to themselves – and could even lie down.
But transport has become busier, and there has been a big push, especially by airlines, to fill every seat.
On some ScotRail trains, such is the current shortage of carriages, things have gone well past that.
The operator is so lacking in rolling stock on the Edinburgh-Glasgow main line especially, that it has slashed fares on another route between the cities to try to ease the crush.
Passengers on many other lines across Scotland will also have experienced very busy trains, and not just during peak hours or when major events are taking place.
ScotRail’s problem has been caused by a nightmare combination of delays to both of its new train fleets and the leases of some existing trains expiring, with the carriages transferring to operators in England.
Things are about to get better, but also worse.
A stop-gap fleet of ten electric trains has been drafted in by ScotRail, already nicknamed “Happy Trains” because they appear to smile, and they are due to enter service between Edinburgh and Glasgow in July.
However, travellers on the equally squeezed long-distance routes between those cities and Aberdeen and Inverness will be dismayed to have heard this week that the first of a fleet of refurbished 40-year-old InterCity trains to provide much-needed extra seats has been delayed for at least two months and is not expected to be in service until July at the earliest.
But from my experience, some passengers are not helping matters on trains where seats are at a premium.
Last Sunday, when Edinburgh-Glasgow main line trains are reduced to half-hourly, was a case in point.
Boarding a train at Falkirk High, there were no two spare seats together in the carriage designated for cycles.
But this wasn’t just because passengers had understandably spaced themselves out across the available space, or, far less understandably, had plonked their bags and coats on adjacent seats, presenting both a physical and psychological barrier to others looking for somewhere to sit.
What caught my eye was the most extreme case of “manspreading” I’ve come across, involving a group enjoying a beer.
Three of them were sitting round a table of four seats, but a friend had decided instead to hold forth, swigging from a large bottle, while standing leaning against the table opposite, effectively warding off anyone else from those other four seats.
There was a different problem on one of Glasgow’s busiest cross-city routes, which, inexplicably, has only two trains hour on Sundays compared to eight on every other day.
There, passengers travelling three stops or more stood blocking the area round doors despite there being seats to be had – and they weren’t to be moved.
Families with pushchairs and bikes just had to fit round them as best they could.
Come on folks, budge up and be considerate.