AN ELDERLY man, a pregnant woman and someone with a disability get onto a crowded bus.

No, it isn’t the start of a joke in really bad taste. It’s the sort of hypothetical situation I’ve been mulling over this week.

I’ve spent the last few days travelling by public transport, and mostly enjoying the break from driving. (Those of us on the main ‘corridor’ through Bournemouth and Poole no longer need to faff around with timetables, but can just turn up and wait no more than a few minutes for a Morebus.)

But on the few occasions when seats have been in short supply, I’ve taken to wondering about the unwritten codes that dictate who gets to sit down.

When you think about it, this simple, everyday situation brings together all our anxieties about changing attitudes to gender, age and ability, and then adds our very British desire not to cause offence or be embarrassed.

I suspect we have outlived the era when a man routinely gave up his seat for a woman even if she was young and able-bodied. Some people will see such a gesture as old-fashioned at best, and creepy or sexist at worst. I like to think displays of courtly manners can come off as charming, but certainly a man is unlikely to be shunned for staying in his seat under such circumstances.

But beyond that, what criteria should I – a middle-aged, able-bodied man – follow? Most public transport has signs asking passengers to give up certain seats in favour of elderly people or those with disabilities, which surely should be expected anyway. But that’s a fairly rough and ready rule.

At what stage do you start being elderly? Surely it doesn’t start at bus pass age – or even pension age, which of course is going up. What if you give up your seat and mortally offend someone who is no older than Sir Mick Jagger? Or if you estimate someone to be decades older than they really are?

And how do you balance age against disability, especially when someone’s disability might not be obvious to the observer?

It’s common sense that you should give up your seat to a pregnant woman. But what if you make a mortifying mistake by assuming someone is expecting when she’s just a little fuller of figure?

I’m personally inclined to offer my seat to a parent with small children, since they need to keep a close eye on the kids (and they’re also likely to be harassed and exhausted, frankly). But the etiquette guide Debrett’s recently decided that adults should not routinely give up their seats to children on London’s Tube.

And how do you rate all these different groups against each other?

Wheelchair user Doug Paulley took his case all the way to the Supreme Court, and won, when he was refused entry to a FirstGroup bus because a woman with a buggy refused to move it for his chair. You could certainly understand his anger at being denied access to public transport. Should we consider the legal precedents when travelling?

As I pondered all this, I started to mentally draw a diagram, like those World Cup tournament charts they give away in national papers.

In Group A, pregnant woman meets older man. In Group B, elderly woman takes on younger man with baby buggy. And so on, until you find the person everyone else should stand for.

As with all matters of modern manners, it’s a minefield. With our British hang-ups about doing the right thing and behaving politely, it’s surprising that passengers on public transport aren’t constantly bobbing up and down like the characters in those Whack a Mole games.

Of course, some people just avoid the whole subject by staring harder at their phones and ignoring the world.

I’m tempted to take the pressure of myself by quitting the whole business. I’ll stand if there’s any prospect of the service being overcrowded.

But how many years have I got before someone offends me terribly by deciding they should offer me their seat?