This is the fourth part of a series on urban design gone wrong. For part one, . For part two, . For part three, click here.

There is perhaps no more divisive term in our cities than that of urban density. Often measured in "dwellings per hectare" or "people per hectare," for some it can evoke our greatest fears about what our neighborhoods might become.

For others, higher density is an environmental imperative to manage the environmental crisis. For yet others, higher densities hold the key to "happier cities."

But there is less consensus than we might think about density. Where does "density" end and "overcrowding" begin?

Fear of density

Many trace our fear of density (at least in Anglo-Saxon countries) to memories deep in our cultural imagination that go back to the overcrowded slums of the booming cities of the Industrial Revolution â?? from the "cattle sheds for human beings" of industrial Manchester, to the reeking cesspools of New Yorkâ??s "Five Points" slum.

It is here that clarity becomes most important â?? to distinguish between the density of people within a building and the "building density" itself.

Rethinking density

Just like reading the growth rings of a tree trunk, we can often deduce when a neighborhood was built by the shape of its streets and density of its form. Tighter-knit, denser neighbourhoods are likely to have been built in the early 20th century, while the winding cul-de-sacs that we are more likely to see today are of a much more recent vintage in our urban history.

That is because from the 1960s onwards, the era of the personal motor car caused an explosion in our urban form. Those who could afford a car took the opportunity took the chance to flee the domain of "Gangs of New York," and the shape of our cities rapidly became a sprawl of lower-density suburbs connected by highways rather than pavements.

There was no need for walking to work or to the shops, so there was equally little need for tightly packed streets and squares. Because of the scale of house-building in the post-war era in the U.S., this more sprawling form now dominates our landscape today. But we are becoming increasingly aware of the unintended consequences of low densities â?? for our health, community and the environment.

Indeed there are signs that the tide is turning again, and that todayâ??s neighborhoods are pivoting back toward higher densities. This is partly because of the need to reduce auto-dependence and the environmental degradation it contributes to, by creating walkable neighborhoods clustered around public transit stations. It also comes amid rising evidence that denser neighborhoods â?? despite our deep-seated fears â?? might be more fulfilling places to live.

Getting density â??just rightâ??

However simply increasing the minimum density requirements will not make for better cities. Just as there is "good density," there is also "bad density," or "cheap density." In fact, architect Lloyd Alter proposes that we aim for "Goldilocks density" â?? "not too high or low, but just right."

The problem is that density is so emotive that it is often misunderstood. When we think of high-density neighborhoods, we might think of the tightly packed glass high-rises of Hong Kong or Singapore. Many contest that "you canâ??t build a community vertically." However, the biggest mistake we make is conflating density with forests of high-rise towers.

On the contrary, density need not be monolithic and alienating. The concept of floor-area ratio helps us to understand how a well-designed 5-story building might achieve equal density to a tower block. This helps to explain why some of the densest neighborhoods in Europe are some of those best-loved by American tourists â?? Barcelona, Berlin, Amsterdam, Stockholm.

The battle over density can too often lead to battle lines being drawn by die-hard density advocates and suburban "nimbies" fighting for their lawns and generous plots. Density is never a silver bullet. However there is a more sensible debate to be had about how to build neighborhoods that avoid the worst excesses of sardine-tin neighborhoods, but that celebrate the lifestyles that living a little closer together allow us to live.

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