Maybe it's to do with regional styles of architecture and human-scale buildings. Maybe it has more to do with trees, water features and a hierarchy of private, semi-private and public spaces. It is hard to define but easy to spot. When parking your car at one of the many DIY warehouses that line London's arterial roads, you know it's not there.
For 5,000 years, we built walkable cities. It was only after the first world war, when cars became cheap and reliable, that we started to build at low densities, with land uses separated into zones. Visionary architects like Le Corbusier and Frank Lloyd Wright wrote about whole cities built this way. They looked great on paper, sketched as if viewed from an aeroplane, but places that got built along these lines turned out to be either windswept concrete jungles or sprawling car-dependent suburbs. The je ne sais quoi had been eradicated.
Planners today haven't quite figured how to get it back. Go to the historic centres of most of our towns and cities and you'll find a lot more people walking, cycling and using public transport than in the recently-built housing estates, office parks and retail complexes that surround the same towns and cities. It seems the more recently your corner of town was built, the more likely you are to do lots of driving. With the exception of a few urban regeneration schemes, most of what gets built today is low density, single use and car-dependent.
All of this, of course, is going to require a sea change in the way we think about planning. The function of city planners has evolved to cover the planning of road networks and the approval of new developments against codified sets of rules.
The effect of this is that the actual design work is carried out by property developers, who for the most part prefer to stick to whatever formula worked for them on the last 20 housing, office or retail developments. Today, there are no visionary planners. Or if there are I haven't met them.
So where does that leave building services engineers? For years we've been using tools like BREEAM to evaluate the carbon footprint of our building designs. But this only scratches the surface of the walkability problem. It's quite feasible to plonk a superstore down by a motorway interchange, miles from the nearest houses, and get a BREEAM excellent on account of a couple of bike racks at the side of the massive car park. We need a system that quantifies the carbon footprint of city plans and compares them with the alternative. And there always is an alternative.
So we need to develop an energy rating for cities. If we had one, we'd probably find that most new developments take the form of D-rated suburbs. In the absence of such a method, there is a simple test you can carry out yourself in your own neighbourhood. I call it the ice-lolly test.
Can a ten-year-old walk safely from your house to a shop, buy an ice-lolly, and get it home before it melts? If so, you probably live in a walkable community.
David Bleicher is a research engineer with BSRIA:
Tel: 01344 465600
or email firstname.lastname@example.org