This is the second part of a series on urban design gone wrong.

Woe the fate of the poor urban designer. Forced to wander the streets of our cities, painfully alive to all of the errors of our design history, the sort of things that most of us walk past with barely a glance.

In this series of articles, we will be looking at a few of the things we have been getting badly wrong when designing our cities, and that we are now scrambling to rectify. That will mean throwing light on a series of often unglamorous details and the surprisingly powerful role they play in our everyday environment.

This week: cul-de-sacs.

What are cul-de-sacs?

Elegantly derived from the French for "bottom of a bag," a cul-de-sac is essentially any street that leads to a dead end with only one way in and out. They form part of our road geometry, and real cul-de-sac aficionados will know they even have internal variation, including the "dead end" and the "bulb."

Pity the town of Culdesac, Idaho, named for its location at the dead end of a railway line.

The cul-de-sac came of age hand in hand with the rise of the suburb, as urbanites fled from the "crime and filth" of the city, empowered in the postwar era by the possibilities presented by the onset of the motor age.

The unintended consequences of the cul-de-sac

They were designed with the best intentions in mind — seen as safe spaces for children to play. The American Dream in all its curvilinear glory. But despite wild popularity in the postwar period, they have since become the bete-noir of urbanists. Charles Montgomery, who wrote a book on "Happy Cities" has accused them of being bad for our health.

The problem is in the road geometry that cul-de-sacs produce. Since the postwar era, our "dendritic" roads are laid out like the trunks and branches of a tree. We drive back from the office along a (usually congested) arterial trunk road and then we gradually filter off along ever finer branches into our idyllic cul-de-sac neighborhoods. This was a departure from the grid-iron patterns of pre-war cities.

Cul-de-sacs were only made possible by the technological advances in the motor industry and the age of the personal motorcar, which freed residential settlements from having to nestle themselves along public transport routes — freedom! Unfortunately, cul-de-sacs are a good example of how street design can be reflexive. Made possible by the "freedom" of motor cars, this new street geometry was set in concrete and actually had the effect of restricting our freedom by forcing everyone into the car.

Understanding the impact of cul-de-sacs requires a quick review of the concept of permeability — how urban form can permit or restrict movement between places. While a "permeable" neighborhood will offer the walker, biker or vehicle a number of options for getting from A to B, in a cul-de-sac layout the kind of distance that should make a short walk often means a lengthy circumnavigation through a tangle of roads.

What is important to understand is that cul-de-sacs do not reduce traffic, they merely displace it. In fact, because of the reasons outlined above, they actually generate traffic — which makes sense when you look at the traffic-clogged arterial "feeder" routes.

In defense of the cul-de-sac

Cul-de-sacs are not at their height of popularity among urbanists. However, they clearly remain popular among homeowners.

Some researchers have also stood up for the cul-de-sac, which adds a bit of complexity to calls to eliminate them from American street geometry. Some have found that cul-de-sacs can, in fact, boost social cohesion. This is particularly so for "bulb" cul-de-sacs, where houses are splayed out around a central car turning circle like the rays of the sun. This is largely because of the natural panopticon they create around children playing, and the fact that strangers immediately stand out in a street with no other destination than its houses.

However, when it comes to security, we come up against the issue of perception and how good urban design can actually be counterintuitive. While tightly interconnected neighborhoods, with corners rather than gentle curves and dead ends, might be perceived as more dangerous, research shows they are actually safer. Just as our setback "moats" might make us feel safer, but will not in reality make us safer.

It is perhaps telling that dead-end streets were first used by the ancient Greeks and Romans as a defensive tool for cases of a siege of the city. Are we building neighborhoods or fortresses?

How to set it right

For structures already fixed in place, this is a tricky task. How do we retrofit a cul-de-sac? One option is through strategically placed paths and connector streets to boost permeability.

However, the most powerful tool might be how we approach new settlements. Promisingly, planners and urban designers are starting to go back to the roots of cities when designing new urban layouts, by requiring more sustainable lattice-like urban grids or even creative riffs on the grid like the "liquid grid."

Conclusions: Tackling 'leave-me-alone-ism'

Cul-de-sacs certainly divide opinion. The suburbs have been described as icons of a new "centerless world," and the cul-de-sac has an important role to play. They are a spatial expression of the centrifugal forces that have held sway since the 1960s, literally flinging us into our cul-de-sac nets at the end of the workday.

The suburbs too are victims of stereotypes — there are plenty cul-de-sacs that nurture strong communities. While individually they may be harmless, it is their cumulative effect on the wider neighborhood that wins them their enemies.

Ultimately, the major problem with suburbs is their problematic association with freedom. Fleeing to the suburbs is not the same as fleeing to the radically libertarian "seasteading" experiments floating in the ocean — they are not a radical assertion of freedom but a search for a compromise where people want enjoy the best of both the urban and rural life. However, in an age when environmental problems stack up, that compromise is looking less and less viable.

The implication is that we might have to do away with the illusion of the "country in the city" and what blogger Chris Norstrom describes as leave-me-alone-ism, and instead begin to learn to enjoy sharing space more closely with those around us. We might begin with street design.