In the esoteric world of car design there is a useful term called 'sink time'. This is the length of time that it takes for a new feature, like the van-like recessed hatchback detail on the Mark 4 Golf, to stop putting people off and start turning people on.
Clearly for marketing folk the duration of 'sink time' is rather important - hence our Battle of Britain airfields full of Rover 75s - but it is more than that. It is a way of talking about innovation without bringing down the witless crows of sustainability in force.
Unfortunately the problem with trying to apply the concept of 'sink time' to the built environment is that, by automotive standards, it is more like 'sink cycle time.'
In housing, for example, no feature is ever really new because no feature ever really goes away. It is as though the Mark 1 Golf, instead of rolling gently into retirement circa 1983, had gone on to complete another 50 laps before being unveiled, unchanged, to an apparently brain dead public as the Mark 5 Golf, its 'sink time' clock wound back to zero.
Housing densities make the point even more forcefully. The subject is a matter of almost obsessional interest at present, what with the deputy prime minister being taken to look at 'high-density' housing in Bath - instead of inner-city Leeds - and everybody determined to forget about the era of 'High Density Low Rise' council housing in the 1960s (whose 'sink time' turned out to be 'sink estate time').
And even that didn't stop it coming round again. To judge by the problems experienced by two of the seven 'millennium villages' so far, the only people who remember what was wrong with hdlr are the people who still live in or near hdlr estates.
All the politicians and professional consultants involved in the recycled version - yet another new Labour 'Beacon' policy - appear either to have been born yesterday or to have had their memory banks wiped squeaky clean.
'Mark 1 Golf?' They scratch their heads. 'Never heard of it, squire.' 'Prefabrication? Never heard of it, my son'. 'What you want is one of these new public sector 'Universities by Example', they're the ticket.' As for living in a Millennium village - 'similar to Poundbury' as they always say. Why don't they add that Poundbury itself is 'similar to a Welsh mining village' only without the mine?
What underlies this mindless recycling of ideas? According to John Gray in Enlightenment's Wake it is a caused by the climate of incessant change promoted by market economics that nullifies the importance of precedent and destroys the authority of the past.
So far so good. But what happens when the past is forgotten is that the future takes its place. Because nobody can remember what worked or didn't work yesterday, old ideas are perpetually dusted down and put forward again, no matter how incongruent with present realities they clearly are.
Thus one lot of politicians grasps at our obsolete nineteenth-century rail transport system - recently made to look more enticing by cutting it into slices and splattering it with eclectic livery - and trumpets it as the answer to the problems of our obsolescent twentieth-century road transport system.
The next lot then seizes upon the twentieth- century road transport system - recently made more enticing by its conversion into a cash register for governments, mayors and local authorities - and denounces it as the source of all congestion, pollution, death and injury.
Left out of the loop forever is the 'sink time' thought that the only real answer lies in distribution and communications systems capable of reducing transport activity to eighteenth-century levels.