Category Archives: Street Network Evolution

the evolution of urban street patterns over time

The Vancouver Fused Grid

Visiting Vancouver, BC, I looked for what some called the “Vancouver Fused Grid”.

It is an example of street network modification that follows the precedents of traffic calming in Berkeley and Seattle in the late 1970s. Nested near the downtown core and being densely built up, Vancouver West, like other neighbourhoods in many cities, gave up on the 4,000 –year old pure grid and modified it to protect the peace, quite and safety of its residents. They did it for quality of life not for cosmetic reasons.

Twenty years later, this neighbourhood shows clearly the benefits of the fusing the merits of the old grid with the advantages of disallowing through traffic while permitting unrestricted pedestrian and bicycle access. The positive outcome of these modifications is confirmed first by the fact that more of the same devices are being installed and by a study of the prevalence of collisions before and after the modifications that shows a substantial reduction. The elements of the Vancouver Fused Grid are:

  • Green Streets” or “Green Connectors”. These are segments of streets that have been closed off to cars. Over the years, vegetation, pavement treatment and street furniture have turned these segments into places for people to engage in casual talk while resting for a few moments to look at billboard announcements.
Entrance to Green Street Connector
Entrance to Green Street Connector
  • Diagonal diverters. A raised, paved and landscaped platform connects diagonally the two opposing sidewalks, thus disconnecting what would have been a direct route for cars. Pedestrians, can jaywalk, a previously illegal activity, without danger. Bicycles can go straight through using specially provided gaps in the platform. In geometric terms, diagonal diverters turn a cross intersection of a grid into a loop.
Diverter connects opposite corners of a grid cross interesection
Diverter connects opposite corners of a grid cross interesection
  • Traffic circles. Unlike the large diameter roundabouts, traffic circles are small circular, raised platforms placed in the centre of an intersection making it impossible for cars to continue on a straight line. Far more effective than four-way stops, they permit a smooth flow of traffic while, removing the risk of running the intersection, slowing traffic down at the intersection and maintaining a direct line of movement for pedestrians without increasing the crossing distance. In terms of geometry, the insertion of a circle in the centre of a cross intersection effectively turns it into four simultaneous or coinciding T-intersections, as does its larger relative, the roundabout.
Traffic circle with greenery and tree
Traffic circle with greenery and tree

Why has this been called the Vancouver Fused Grid? Because it incorporates two basic elements of the fused grid network layout: first the discontinuous car network and continuous pedestrian movement (or giving pedestrians an edge over cars.) Second, providing places of rest and tranquility, where danger and tension dissipates and nature or people become the object of attention.

A neighbourhood model showing the network layout that differntiates paths from roads
A neighbourhood model showing the network layout that differntiates paths from roads

The Vancouver Fused Grid responds to the natural and frequent question when planners and residents encounter the model: Can it be applied to existing neighbourhoods? the Vancouver example gives an affirmative answer.

Fused Grid Worlds – Mutual discovery

 

 Looking around I realized that there exist other inhabited Fused Grid worlds

Hello Simtropolis!

 Players of SimCity 4 discovered the Fused Grid and are using it to build efficient cities that, apparently, work well, when the points are counted.

 Take a look at their work, full of good ideas of how to use the Fused Grid model to build good cities. Even within the confines of a game with limited library of buildings and elements to populate the space, the screenshots, particularly this one, still give a fairly realistic picture of how a neighbourhood and a district would work….. until the first subdivison is build in Alberta and the first city is built in China.

 

New words for new layouts

Recently, I found a new, useful term that was introduced in the discussion about street networks: filtered permeability. Its author Steve Melia  explains: “It is the concept… that networks for walking and cycling should be more permeable than the road network for motor vehicles”

 

This new term helps place many known street patterns into a spectrum, from unfiltered to filtered permeability, by degree of exclusion. For example a steep, stepped, narrow street in an island village is only permeable to pedestrians and it excludes ALL other means of transport. Narrow streets in Old Fez city are permeable to pedestrians and animals  but impermeable to carts or cars. Some city streets, though in principle permeable to pedestrians have turned impermeable because of the dominance of cars. By contrast, a Highway is fully permeable to all motorized vehicles but not to bicycles or pedestrians (by decree).

For planning new neighbourhoods, this means we can consciously design a degree of permeability for each of the streets in a layout. See how this is accomplished here.

City Council gives the go ahead to a Fused Grid subdivision

40 acre (16 ha) portion of The New DevelopmentThe Fused Grid model for planning new neighbourhoods will soon be part of Calgary’s map. City Council approved unanimously the Saddleton (64 ha) development plan on April 14, 2008 with one Alderman saying “..he was pleased to see the Fused Grid concept come to fruition.” Since 2003, CMHC has researched, developed and promoted the Fused Grid option for laying out subdivisions for its many benefits for homebuyers, municipalities, developers and the environment.

This approval will please all those who were have worked hard to promote it and will add confidence to those who are considering its application. It is no longer simply a theoretical construct it is a neighbourhood plan heading for construction.

Where did the grid come from?

There is hardly any city in North America that does not have a central area with a grid pattern. Many South American cities share this feature and a good number of European ones as well. This universal use of the grid as a stencil to plan new cities is intriguing.

Where did this stencil come form?
It is often called the Hippodamian grid after the Greek town planner who used it to lay out his home town after its destruction by invaders. Yet we do know that he was not its inventor. Many early cities appear on archeological lists that precede Hippodamus and his most celebrated layout of Miletus.
If he borrowed the idea from other cities that he happen to visit, why did he think it was a good solution? After all the majority of ancient cities grew by accretion, organically, and did not show any signs of pure geometry, including the most celebrated city during his lifetime –Athens – in which he had admirers and critics.
That he was, allegedly, a Pythagorean disciple may explain his proclivity toward geometric forms but does not explain his decision to borrow and apply this stencil.

The first evidence of grid-like layouts for settlements in Mesopotamia and the Hindus valley goes back to around 2500, that’s about two millennia before Hippodamus’ time and as far back removed as we are from his invention/adoption.

What then brought the first grid-like layouts into existence? Most likely the same forces that shaped the highway network of this century – a new means of transport.
According to a chronology of events listed in a recent book, the agrarian period in history that brought about permanent human settlements started at about 4000 BCE and along with it came animal transport. Five hundred years later, around 3500 BCE, wheeled transport emerges. One thousand years after it surfaced the first grid-like settlement layouts appear.

Like the car, early wheeled transport must have had a difficult time adjusting to the organic, maze-like layout in which it was introduced. Carts and chariots cannot negotiate sharp turns and move best on a straight line. Frequent t-intersections or turns make the tip long and arduous. In addition, carts and particularly chariots are normally drawn by two horses. Two attributes of the old street layouts would have made access to places either impossible or very difficult: the width of streets and the maze-like pattern. A third would also seriously hinder access –slope. Many settlements that were built on hill sloes and tops, for defense purposes would be unable to accommodate the new technology. Naturally, when the friction between existing street patterns and new transport means became intolerable a new street pattern emerged. It would not be hard to see why Hippodamus would have readily adopted it.

To see striking examples of streets that are inaccessible to carts and chariots look up Fez the Old city in Flikr or my postings of Greek island streets also in Flikr
The book that lists the chronology of transport revolutions is: “Maps of time: An Introduction to Big History” by Cristian, D. (2004)

New Research Evidence

Looking around, I found research that shows before and after comparisons of physical activity and, consequently, health. This one is from Britain and it looks at neighbourhood traffic calming measures. It confirms empirically their positive impact on physical activity. It was done by David S Morrison, Hilary Thomson and Mark Petticrew under the title:
Evaluation of the health effects of a neighbourhood traffic calming scheme

 In its summary it says:

“Health impacts of neighbourhood traffic calming

This study provides support for the proposed theoretical links that the health impacts of neighbourhood traffic calmingschemes may go beyond accident and injury reduction. Selfreported physical health, observed pedestrian activity, andtraffic related nuisance improved in the local population aftera traffic calming scheme was built in the main road. “

You can find the full article at: www.nice.nhs.uk/nicemedia/pdf/word/Transport%20evidence%20review%20summary.doc – My thoughts on this are:

Here is another example that shows that the street patterns we inherited do not produce the best outcomes for neighbourhood  residents; they need to be modified to do that.
So, let’s do it right the first time. Get the traffic out of the neighbourhood by design. The fused Grid does that. It creates a 64 ha area that is free of through traffic and provides a dense network of pedestrian paths on and off road.