Sex in a public place is against the law. You probably know that. But do you want notices in the high street or park, or recorded announcements on trains, informing everyone of the fact? No. Though some may be unaware of the law, some break it, and few would be confident we knew the precise legal definition of a public place, the prohibition is presumed, not advertised.

It could be – like the incoming smoking ban in England, which will soon require churches and all other enclosed public spaces to place “No smoking” signs at the entrance. As correspondents to this newspaper have pointed out, smoking has been singled out for special notices: other things you may not do in or out of church have not.

Why not sex, for instance? We could design little figurative icons: maybe a horizontal couple (or the soles of four feet?) within a red circle crossed by a diagonal bar. But instead we presume that citizens know about such bans; or, if in doubt, will find out, though the legal definition of “a public place” is never advertised. This, too, we are expected to know, or guess, or find out.

In fact there is an almost infinite list of things we may not do and places where we may not do them, warnings about which are seldom stuck on iron poles or broadcast over loudspeakers. We may not light a barbecue in a Royal Park. We may not commit murder anywhere at all. We may not grow vegetables on common land. We may not hunt with dogs. We may not steal anything from anyone, ever. We may not obstruct the pavement. Some of these prohibitions are obvious and widely understood; others less so. But obvious or not, the defence that we were unaware of the law will seldom take us far in court: it was our responsibility to find out.

Ignorance of the law is no excuse, they say. Yet with each passing year the bombardment of eyes and ears with admonitions, restrictions and prohibitions grows more intense. The spread of signage has become a serious modern evil: a kind of environmental pollution. “No smoking” signs are almost ubiquitous. Pavements sprout a forest of poles bearing tin plates on which parking restrictions, speed restrictions, access restrictions and warnings about road humps, bus and cycle lanes, uneven surfaces and hazards of every kind, shout their advice. Looped tapes on public address systems endlessly repeat banal exhortations to mind this, take care of that, avoid one thing or remember another.

Some are compulsions with the force of law, some just advisory. Some notices are themselves required by law. Others are stuck up for good measure and the avoidance of doubt: the result of a belt-and-braces policy adopted by many public authorities. In the national mind, our understanding of what does or does not need to be prominently posted has reached a state of some confusion.

It was probably during the late 19th century that logic as to what the public should be told and what it should find out for itself first began to lose its way. Partly to blame will have been the growing appetite – often high-minded – for improving public behaviour through complex regulation. The expanding powers of big, muscular local government with its proliferation of bylaws applying only within certain localities added to the confusion. The slow closing-in of the criminal law on the motorist, so that Mr Toad’s once-open road has over the decades been sown with a minefield of explosive little potential crimes of which highway authorities now warn unwary motorists, has played a big part too.

Of course the case for explicit and prominent notices of a regulation is strongest where there is no blanket prohibition but where rules depend on place or time. It’s easy to grasp and remember a general ban, like the law against arson, but what about bylaws regarding bonfires – where and when they may be lit? Citizens may pass into and out of differently regulated zones, and hope to be notified when they do. With some kinds of regulation an assumption has grown that if it isn’t advertised by a blindingly obvious sign that nobody could credibly claim to have missed, then the citizen is not at fault for ignoring it. In other cases the signposted advice needn’t be prominent, but must at least be findable if sought. In yet others (such as drink-driving laws, hunting laws or the “no dumping” rule) the old maxim that ignorance of the law is no excuse still applies. Yet we do see signs saying “No dumping” and are regularly reminded about drink-driving, whereas the motorist’s obligation to stop for a pedestrian on a crossing is never posted beside the flashing beacons.

There is no rule of thumb (and so far as I can see none of jurisprudence either) to guide us. Meanwhile, there continues a slow spread of the working assumption that “best practice” requires regulations to be clearly and prominently advertised. The spread contains the seeds of its own expansion because the more widespread become the signage, the stronger becomes the argument that where there is no sign, the citizen can assume there is no regulation.

The assault of signage on our environment contains the seeds of its own expansion in another way too. The more warnings and notices there are to clutter sound and vision, the more the law of diminishing returns applies, as each struggles with a sea of rivals for our attention. All the higher animals have a life-preserving capacity to “screen” out what has become routine, focusing instead on what is unusual in a scene.

There have been reports recently of research showing that “DayGlo” orange is losing its capacity to thrust itself to the forefront of modern man’s attention because we’re now so used to it. So the DayGlo must glow brighter, or the sign grow bigger, or the announcement be louder or repeated more often, if none is to be crowded from our attention by competing notices. Too much is already tugging at the sleeves of our sensibilities, and as the transmitters turn up the volume knobs, the receivers (us) turn ours down.

But this dynamic works in reverse, too. The fewer signs exist, the more we notice those there are. As signage grows more discreet we become more alert for it. As signs become smaller we search harder. If you could hear a pin drop, then you can. And, as with our physical attentiveness, so with our attention to the rules: the less confidence we feel that they will be prominently advertised, the more trouble we will take to find out what they are.

It is time for government to take a planned, across-the-board decision to repulse the advance of signage. It should be a national decision. At its heart must be the concept of presumption. Wherever this is realistic, the public should be presumed to know the rules or, if in doubt, take responsibility for finding out. When I’m on a narrow country lane, I presume – and the local highways authority may presume that I presume – that the speed limit is 30mph. This is the logic we need in our fightback against signage.

There is an obvious place to start and a logical reason for starting there now. For as long as smoking prohibitions varied from location to location, determined by those in charge, there was an obvious case for posting smoking bans on notices. But the new law, whatever else it may represent, represents a massive simplification. The ban applies to all enclosed public spaces and workplaces. The definition of an enclosed public space and of a workplace is broadly comprehensible to almost everyone.

So there need be no “No smoking” notices in buses, trains, cinemas, pubs or offices – let alone cathedrals. None. Literally millions of these tin and plastic pimples on our ambience can be scrapped overnight.

You don’t smoke at work or in a public enclosed space. You don’t pull your trousers down. You don’t start a bonfire. You don’t make love.

It’s that simple. And there is no need to put up a notice.

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