While walking through Sydney late at night recently, I encountered a weedy-looking man in a high-visibility jacket standing on the foreshore underneath the Harbour Bridge. He’s there, I’m told, to ensure that Johnny Terrorist cannot blow this symbol of Australian pride into the water.

So, let us examine his chances of success. If a band of Muslim extremists determined that Australia should be hit next — and, let’s be honest, this is a very big “if” indeed: it’d be like attacking Andorra — then yes, the Harbour Bridge would be a juicy target.

But, of course, it is actually quite difficult to blow up a bridge, especially one as sturdy and enormous as this Middlesbrough-made monster. Many hours would be needed to plant charges and, as a result, a large team of demolition experts would be necessary. Can you imagine how much planning would be required for such an operation?

Months. Years, maybe. And I’m sorry, but I doubt very much that a highly trained team of Muslim demolition terrorists would get to Sydney, with all the explosives necessary, after a year’s planning, and then say, “Oh, no. There’s a man in a high-visibility jacket patrolling the foreshore. That’s torn it.”

Of course, they could attempt to destroy the bridge by crashing a plane into it. But to prevent this, what you need is a battery of missiles on the towers. Not a skinny little man, in a yellow vest, on a beach.

Of course, we know why he’s really there. He’s really there so that if the bridge is destroyed by terrorists, the authorities can appear on the television news and say they had taken all possible precautions. Plus, if you employ a security guard, then I should imagine that your insurance premiums are going to be significantly lower.

This is probably why so many companies use security guards these days. It must be, because when it comes to preventing a crime, they are pretty much useless. No, really. If you are planning a heist, job one on the list of things to do is “take out the guard”. He is therefore not an impenetrable wall of steel; he’s just a nuisance.

And he’s not just a nuisance to the people planning to hit him on the head. He’s also a nuisance to the thousands of people who legitimately wish to enter or leave the building he’s supposed to be guarding.

At the office where I work, everyone is issued with laminated photo-ID cards that open all the barriers and doors. It is quite impossible to make any sort of progress unless you have such a thing about your person. But even so, every barrier and door is also guarded by a chap who, in a fight, would struggle to beat Christopher Robin. One looks like his heart would give out if you said “boo”. Another has a face that’s so grey that, in some lights, he appears to be slightly lilac. I cannot for the life of me work out what these people are supposed to achieve, apart from making the lives of normal people a little bit more difficult.

How many times have you been stopped on the way into an office block by an octogenarian who asks all sorts of pointless questions even though he can plainly see that what you have in your hand is a sheaf of papers and not an AK-47 assault rifle? And how many hours have you wasted while his colleagues input every single detail of your past life into a computer so that you can be issued with a visitor’s ID, which you are told you must hand in when you leave? But you never do.

If the system worked, and was necessary, they would assume as the office closed that you were still in the building somewhere, possibly wiring up a bomb. So they’d call the mobile number you’d left. And go round to your house. And contact your next of kin. But they don’t, which must mean the visitor ID card system is a waste of time.

So are the men who operate it. Recently, for instance, one asked me — by name, incidentally — for my security pass, which had just opened the tank trap that stops Osama Bin Laden getting into the car park, and stood for several moments comparing my picture and my face. Eventually, having determined that they were one and the same, he waved me through ... which is odd because at no point did he ask why I had two 12-bore shotguns slung over my shoulder and a pocketful of ammo.

Not that I would have needed such artillery to take him out. A light breeze would have done the trick just as well.

I can understand why oil company executives might need security while on a fact-finding trip to Baghdad. And I can see why you might need to employ a burly chap if you run Shafters Nite Spot in Sheffield. But a firm of accountants? In Holborn? No. That’s either a sop to the insurer or, more likely, corporate vanity.

There is, however, a way that you and I can overcome the robots who guard the world from nothing at all. It’s easy to become flustered and cross with the frail old man in the yellow vest as he sticks a spanner in the smoothness of your day. But this will achieve nothing, since he is — as he will remind you — only doing his job.

Instead, just walk past them, smiling your best vicar-at-the-end-of-the-service smile and explaining that everything’s okay. It always works. They won’t actually run after you; they don’t have the breath. And they won’t physically get in your way, because there’s bound to be a health and safety rule that prevents a security guard from rugby-tackling a grown businessman.

Mostly, though, they won’t want to stop you because you were smiling and you were being pleasant. Nobody wants to become physical with someone who’s charming and pleasant. It’d be the same as punching Jilly Cooper.

Frighteningly, though, I suspect that this super-nice approach would also work for terrorists in Sydney. If they just walk past the weedy-looking chap with a smile and a couple of pleasantries, he’ll still be smiling right back when the entire bridge comes crashing down on his head.