Almost unnoticed, an old tradition of political dissent and protest is being eradicated in Britain. Edinburgh and the environs of Gleneagles were taught as much during the G8 chatter. The new rules, without benefit of any legislation whatever, are these: think what you like, say what you like, sign those petitions to your heart's content. But approach the Queen's highway in any manner that appears organised and you will be regarded as a mob.

Almost unnoticed, an old tradition of political dissent and protest is being eradicated in Britain. Edinburgh and the environs of Gleneagles were taught as much during the G8 chatter. The new rules, without benefit of any legislation whatever, are these: think what you like, say what you like, sign those petitions to your heart's content. But approach the Queen's highway in any manner that appears organised and you will be regarded as a mob.

To my knowledge, not a single politician of significance has bothered to ask whether 2000 police in riot gear, batons drawn, crash teams, cavalry and choppers on standby, were required to quell environmentalists at Heathrow over the past few days. No-one I know of has asked whether the policing of sundry other associated "events" in the south of England was remotely proportionate. A decision was made - by whom? - to protect a business enterprise from people possessed by the quaint notion that the planet needs saving.

It's a big business, admittedly. A friend in the travel trade told me not so many years ago that shifting humans from A to B for the purposes of leisure is these days the planet's biggest industrial enterprise. Heathrow, despised and unloved but forever expanding, is one of the largest cogs in the machine. Some people, not least the ones who have been facing drawn truncheons in order to make the point, believe that the airport's very existence is therefore a mistake. They may be right.

Actually, I don't seriously doubt them. I apply two makeshift criteria to arguments over climate change. The first involves my strictly limited understanding of the disputed science. The second, inspiring just a little more confidence, depends on a knowledge of the sceptics, their history and politics. Begin with George Bush and proceed from there. I would not, under any circumstances, buy a used planet from these individuals, or trust a word they have to say.

The Heathrow protest was justified, then, much as the employment of the police as BAA's private security force was unjustified. The Spanish-owned operator has become fond of saying that travellers - the firm's customers - "have rights, too". True enough. But how does one balance the right to buy burgers and over-priced tat from an airport mall, while en route to yet another lousy mall, with the survival of the plan-etary ecosystem? As with the use of truncheons on noisy vegans, a sense of proportion might be missing.

BAA is very keen to create yet another terminal - apt word, in the circumstances - at Heathrow. That would increase the number of people-processing palaces to six, and the number of flights to 720,000 annually. Will anyone, anywhere outside the company boardroom, be grateful?

It's the sort of simple, boneheaded point of which I have become increasingly fond. My colleague Harry Reid wrote eloquently of the Heathrow nightmare not so many days ago, and I attempt only to amplify the point. What is the public benefit of any enterprise from which the public, in its living and breathing form, does not actually benefit? Enter terminals one to five and your private ecosystem is already at risk. These are, as Harry described, hellish.

Yet will we give them up? The Heathrow protester who lectures me on my carbon footprint will get no argument. So what would the response be if I said, first, that the free movement of peoples is an important, hard-won right? I could add that cheapish travel has expanded the horizons of the working people of my generation in extraordinary ways. I could proceed to say that putting flying beyond the economic reach of ordinary folk and handing it back to the rich and careless is unjust, to put it mildly.

Then, possibly, I could offer something parochial. Would the carbon smears of ordinary Scots not be improved if they were spared BAA's hub system? We all know how it works. Scotland lacks an authentic international airport. In the effort to go anywhere (I generalise, but not by much) you must endure being funnelled through London or Manchester. Often enough, you must fly to one of these great cities just to fly ever onwards. Your carbon footprint acquires a big footprint of its own.

The Heathrow protesters would not approve, I suspect, of Stirling International Airport.

In their peer-reviewed science, that would solve no problems. They would prefer, in fact, that we just said no to no-frills flying and weekend breaks in the arrival lounges of Europe. If their best guesses in the pollution argument are correct, we should pay them heed. Obviously.

So how many people have sworn off foreign travel since it became obvious that human effluent has the whiff of catastrophe? Heathrow wants to expand because demand is increasing. Cheap airlines prosper, despite the historically high cost of aviation fuel, because customers cannot resist the bargains, or give up their hard-earned holidays or deny themselves the chance to see the world. Are Heathrow, the carriers or the customers each immoral?

A purist would give you a blunt answer. I, on the other hand, find it hard to hold a working-class family responsible for the fate of the planet simply because, for the first time in all their generations, they won the right to sit on a Greek beach. This is an issue that environmentalism has yet to address, far less resolve. The muck produced by jet engines is one thing, the impact of mass tourism on fragile places another. But how green is it, exactly, to curtail liberty in Draco's style for the greater good. Whose good?

I could probably live with the hard answers. As it happens, I live in a part of Scotland that is itself accounted fragile and precious. I wouldn't wish to see it messed up. But the Heathrow protest may yet have unintended consequences for those passionate, sincere protesters. They may have reminded the public of the choices available. What is to be said if, in the purest expression of democracy, that public still insists on all the wrong choices?

As we reported yesterday, the decision by the SNP executive to scrap tolls on the Forth and Tay bridges could turn out to be expensive. No-one doubts that abolition is hugely popular. Yet few can be much surprised by the predictable consequences: more congestion, according to an independent report, therefore more pollution. And would the affected drivers wish for tolls to be restored? No chance.

Deep psychology is involved. Who, willingly, a free citizen in a mostly-free country, agrees to be priced off the roads or out of the air? Which Atlas, commuting or holidaying, chooses to shoulder the burden of the planet's fate while transnational industry makes its excuses and the rich go on, oblivious, as before? Personal responsibility is a fine notion until you are the person being held responsible.

The Heathrow protests were impressive, honourable and justified, in this old hand's book. I wonder, though, how a staunch eco-warrior would cope while telling a couple of thousand folk at Glasgow Airport that their fortnight in the sun had just been cancelled for the greater good, and the sake of the planet. I'm lightly green, mostly, but there my resemblance to a cabbage ends.