FOR years urban planners have emphasised the needs of the motorist over those of the pedestrian. Thanks partly to greenery, partly to a greater understanding of how pedestrians behave, and partly to concerns about social cohesion, priorities are changing

London provides two good examples of this shift. On February 1st Exhibition Road, a landmark street near many of the city's museums, is being formally reopened after a three-year construction project to turn it into something that transport engineers like to call a “shared space”. Kerbs have been stripped out, along with the usual road markings, to create a thoroughfare that is designed to be shared by cyclists, pedestrians and cars alike. The idea, adopted from continental Europe, is to create an area which is not just more pleasant for people on foot but also safer because it encourages drivers to pay closer attention to their surroundings.

Less experimentally, big improvements have already been made to Oxford Circus, one of the city's busiest intersections. The junction between Oxford Street and Regent Street sees as many as 40,000 people pass through every hour, and only 2,000 vehicles. Until 2009, however, pedestrians came well down the pecking order. In the language of planners, pedestrians were unable to follow their desire lines, the paths they want to take as opposed to the ones they are meant to. At Oxford Circus, giving rein to people's desire lines has meant ripping out guard railings that hemmed pedestrians in and allowing people to cross the junction diagonally as well as from side to side (a feature known as a pedestrian scramble).

Desire lines can be seen in virtually every public park as the informal dirt trails trampled by walkers as they head off path networks to their preferred destination. They also exist over longer distances within cities. Pedestrians seem to prefer routes that afford most visibility, for example. An experiment carried out by Jan Wiener and colleagues at Freiburg University in Germany gave people a choice of two paths in a virtual environment: they reliably chose the direction which had the longest line of sight.

Similarly, people seem to like routes that head in a constant direction. According to Ruth Conroy Dalton of the University of Northumbria, people perceive routes with changes in directions to be longer than straighter ones, even if they are actually the same distance. Odder still, equivalent routes that have landmarks on them are also reckoned to be longer than routes that do not. That may be because memorising changes in direction and landmarks both require the brain to do more work than a route that simply heads in as straight a line as possible.

These preferences may help explain why it is that some city streets are more crowded than others. Why is it that Oxford Street, for instance, is London's busiest shopping street and not, say, Regent Street or Piccadilly? Tim Stonor of Space Syntax, an architectural consultancy, says that the answer lies in graph theory, a branch of mathematics that studies nodes and the connections between them. Counterintuitively, though, Space Syntax's model represents street segments as the graph's nodes and road intersections as connections between the nodes. The resulting topsy-turvy simulation is then used to chart the most linear conceivable route to join every street in a city with every other street. It soon becomes clear that not all roads are equal. Some are more accessible and integrated than others—which is why Oxford Street is more likely to be walked along than any other street in London.

That has implications for how street layouts can be consciously designed to create areas that are more or less vibrant—more suited to shopping, say, or family living. It can also be used to identify places that are unhealthily segregated. Mr Stonor points out that 85% and 96% of riots last August in north and south London respectively took place within a five-minute walk of a post-war housing estate. Most observers would put that down to the fact that the estates' cheap accommodation draws poorer folk, resulting in pockets of poverty and deprivation whose denizens are more likely to commit crime and engage in acts of vandalism. But Mr Stonor believes that the complex, insular design of many housing estates exacerbates the problem by limiting interactions between people and thus encouraging anti-social behaviour—the exact opposite of what their creators envisaged.

Correction: In an earlier version of this post we said that Space Syntax's model represented road intersections as nodes of a graph and streets as connections between the nodes. The clever trick was, in fact, to turn that intuitive reasoning on its head. This has now been corrected. Apologies.