SECRETS OF THE URBAN SCRAWL

A DAB OF paint highlighting a broken paving- stone here, a chalk arrow indicating an intended wheelchair ramp there: the writing has been on the pavements for some time now, but until recently it was relatively easy for pedestrians to overlook it. The scribbles underfoot were too rare, too discreet, to trouble anyone but the workers for whom they were intended.

No longer. Over the past few years, pavements all over Britain, and especially in London, have acquired a bewildering new sheen. Numbers, letters, half- words, symbols - in red, yellow, green, white and blue - have proliferated like exotic weeds, colonising high street and back street alike. Urban pedestrians who have not yet noticed them should beware: they are probably not taking enough notice of where they put their feet.

What does it all mean? Some streets boast palimpsests so dense that only a hardened textual scholar would feel confident attempting to untangle them, yet the code is actually quite simple. The key to the cipher is a single word: cable. The rest follows logically.

All over Britain, streets are being dug up to allow the laying of fibre optic cables, designed to deliver both television and any number of additional information technologies to people's homes. Around 4 million homes have already been "passed" by cable; by the end of the decade, cable companies hope that there will be more than 6.3 million cable subscriptions - and every subscribing household will have had a road or pavement dug up in order for its cables to be laid. That is an awful lot of digging; and digging is not the simple business it once was. The earth beneath our roads is already packed with man-laid hardware - water pipes, gas pipes, electricity cables - none of which can be expected to benefit from the sudden intrusion of a pneumatic drill. Hence the colour-coding. Before laying the new cables, the engineering contractors employed by the cable companies are required by law to consult any local utilities which might have underground plant in the area. They also use radio detection to enhance their picture of what lies hidden beneath the tarmac and paving-stones: water and gas pipes and telephone and electrical ducts each give off recognisable signals. The underground map thus created is then reproduced on site with coloured paints, by an employee known as a "tracer".

The tracer's glyphs make obvious sense, once you're in the know. Red is power - look for the P between the curving tramlines in the main photograph, and the HV (high voltage) by the straight lines. White, as the BTs imply, is British Telecom; the number of arrows and the numerals indicate how many separate ducts of cable will be found. A yellow stripe (running over the yellow line at the kerb) denotes a gas main. Green is the proposed route of the new cable. As it happens, green is also the actual colour of the plastic casing of the cable, at least in the area of south London where these pictures were taken.

The difficulties inherent in the exercise are eloquently expressed by the green "with care" at the edge of the road. For all their charts and radio signals, the contractors are never really sure what lies below the surface until they have dug it up. "If you've got a very congested area, with lots of metal pipes, the magnetic waves can bounce off each other and make radio detection less accurate," says Steve Hart, an engineer with TE Beach, whose marks these are. "There's so much down there that you have to move very cautiously."

The ephemeral (and biodegradable) street art generated by cable is, in other words, impressionistic, although some might see it as more abstract than that. Either way, it is not without aesthetic appeal. Whether all this painting and digging will eventually yield any programmes worth watching is a matter for conjecture. But, in the meantime, at least it is brightening up a few streets !