Humantransit has a great post on cul-de-sacs and what is known as the “radius of demand”. In short, the post highlights how disconnected street networks make it much more difficult to adequately serve areas with good public transport, because what should be a relatively short walk ‘as the crow flies’, becomes a really long walk to the bus stop/train station because the street network won’t allow you to make the trip in a more direct manner.

This is excellently illustrated in the diagram below. These are two real-life suburbs in Seattle. The red dot in the centre of each circle illustrates potentially where a train station or bus stop could be, with the circle being a 1km radius around it. Nothing unusual there. But what is really interesting is what the blue lines indicate – the streets which are within an actual 1 kilometre walk of the red dot. In the street pattern on the right, because there is a basic grid pattern around 64% of the “catchment” is actually within the 1km walking distance of the middle. However, in the street pattern on the left, because many of the walking trips involve convoluted detours, and the street network is poorly connected, only around 30% of the catchment is actually within that 1km walking distance to the centre. That means that the walking catchments of these two areas, while at first sight appearing equal, are actually vastly different.

This is reason number one why gridded street patterns are very useful. They improve walking accessibility to public transport stops & stations.

Reason number two why gridded street patterns are useful is when it comes to designing effective public transport networks – particularly if we’re trying to create a “network effect“. This is relatively well illustrated in a recent post I did on trying to adapt the network effect idea into a redesign of Auckland’s isthmus bus routes, where I eventually came up with this:

The idea is to create something of a “grid” of service patterns, so that north-south routes are complemented by east-west routes to create a network/web of routes allowing “anywhere to anywhere” travel. This works OK in the centre-west part of the isthmus, where the main street form something of a “super-grid”, especially around Balmoral, Mt Eden, Sandringham and so forth. But in the parts of the Auckland isthmus to the east of State Highway One it is much more difficult to achieve such an outcome, because the main streets do not form a grid, but rather seem to meander all over the place. This same issue also makes it a quite challenging to create a network effect on the North Shore (and undoubtedly also in Manukau and North Shore cities).

In contrast, if we look at Vancouver’s street pattern it’s easy to see how the network effect has worked so effectively: The main streets form a superb “super-grid”, allowing many north-south and east-west routes to operate efficiently, linking together to create a network that truly allows anywhere to anywhere travel. Furthermore, within each “super-block”, the street patterns a simple and gridded, allowing very quick access from wherever you live to both a north-south and an east-west route.

This is why I find it so depressing to see new parts of Auckland have street patterns like this:It’s not the density of recent developments in Auckland that makes them so difficult to serve with public transport, as in fact many recent growth areas are quite high density due to the high value of land. I actually think the bigger barrier is actually the street patterns that we’re seeing – like in the area above. This kind of street pattern discourages walking anywhere (as it’ll take forever because you have to travel so indirectly) and as a result contributes (along with a neglected public transport system) to making auto-dependency inevitable. I’d love to say that there has been a realisation of this in recent years, but sadly the area above is one of the newest parts of Auckland, as is the area between Albany and Greenhithe which is arguably even worse.

Street patterns matter. A lot.

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32 comments

  1. I was reading about some city in the Carolinas in the US which had a shocking pattern, very low density, cul de sacs, lollipops on sticks, a real shocker and they got a guy to ID all the empty spaces and the minimum number of properties to buy to re-establish a grid, to then up density and run PT on… Very interesting…

  2. If you look at Palmerston North, it take a grid shape approach, but on the fringes of town, there are many cu-de-sacs. This is an example of a city that has been effected over time by the changes in planning subdivisions. However the damage is only minor. I was reading this blog earlier from Jarret and I was thinking about where I live now, Tawa. Suburban sprawl and topography has had such an effect on the landscape that getting anywhere is a hassle. To get to the Tawa shops is a similar distance to the Porirua or J’vill shops from where I’m living. The building of the motorway has split the town in half, with further development, there has been poor access to the existing rail services. This poor access plays a strong part in the way Wellington suburbs where created. We are just lucky we got an electrified rail network!

    Living in suburban Wellington sucks!!!

  3. Ironic that people want to live in quiet cul-de-sacs ( where does that word come from anyway? ) to get away from cars and they end up contributing to the problem.

    Anyway i don’t see that cul-de-sacs should be a problem so long as the planners are smart about where the alleyways go.

    My grandparents live in a quiet development where all roads are dead ends. To the station is about 1.1 mile by road,

    http://maps.google.co.uk/maps?f=d&source=s_d&saddr=kt20+5pr&daddr=Station+Approach,+Tadworth,+Surrey+KT20+(Tadworth+Railway+Station)&hl=en&geocode=FWTGDgMd4H78_yk38xMyAeN1SDFqGsyUfAfcrA%3BFfqjDgMdtGj8_yHKth5GYjpAEA&mra=ls&dirflg=w&sll=51.29564,-0.232755&sspn=0.015752,0.036349&ie=UTF8&ll=51.298101,-0.223417&spn=0.031503,0.072699&z=14

    but if you take the alleyways (not shown on the map) it is pretty much straight as the crow flies. Takes me 6 mins to walk as fast as i can without running.

    The only issue is you’d need stable public transport routes. Works fine with train stations but it’s no good if a suburb is developed with alleways to a bus stop and then they re-route the bus miles away.

  4. Looking at the map above I see there are some alleyways that would help alleviate the problem – mostly only between Te Irirangi Dr and Chapel Rd, and not enough of them by a long shot.

    The grid is so much easier and makes more sense. Maybe offset streets by half a block to avoid crossroads intersections which I believe can be bad for crashes, tho.

  5. In Balmoral and Sandringham the blocks are way too big. Also, most of the intermediate streets don’t connect through all the way. (example King Edward Street). So even the neighborhoods with the most potential for PT and car-free living are constrained.

  6. A pure grid can be a bit boring, in that it’s just the same thing over and over and over again. So I think the ways that part of the city is structured works quite well. Unsurprisingly, it developed around the tram system.

  7. I lived in Milton Keynes for a while. It attracts a lot of knockers, but was actually a great place to live. For those not familiar with the city, it is built on a grid of dual carriageways spaced about a kilometer from each other with roundabouts at the intersections. The enclosed areas are called “grid squares”. There are usually only two or three junctions from the major roads in to each grid square, and so there is no reason for traffic to go through a square rather than around it. A grid of roads of this sort is surprisingly hard to congest because there is always an alternate route around any blockage.

    Each grid square is zoned as either residential, commercial, light industry, or CBD and so no one has a supermarket next to them with the associated traffic. This meant that the residential grid squares were surprisingly villagey with homes, a shop or two like a dairy, and often a church or community hall. Planning for the landscape and deliberate landscaping meant that it was rare to see any of the dual carriageways from inside a grid square.

    Lastly, there was a network of dedicated cycleways that would pass under and over the dual carriageways. You could cycle all around the city without encountering a road except at the very ends of your journey when you’d need to cycle along a quiet grid square street.

  8. Great stuff. I see on that map that the area between Te Irirangi and Chapel does seem to have alleyways.

    Alleyways are very tricky, as you note.

    Here in Wellington, paths down the hillsides and connecting switchback roads are absolutely necessary to make walking feasible, and there are lots of them. On the one hand they are rarely fenced, leaving lots of nasty places for nasty people to lurk where you can’t see them, but on the other hand they are in densely populated areas where at night time you are surrounded by people’s houses. I feel pretty safe in them whereas I wouldn’t on a path through a park or a deserted city block.

    A Pattern Language describes this very thing — lots of cul de sacs to discourage cars using your street as a rat run but also lots of connecting paths to encourage foot and cycle traffic. Those paths need good design to work as intended. If you go to Vancouver you’ll see that many of the little side streets off main routes are actually closed to through traffic.

    I feel there’s a possible sweet spot where major routes are on a rough grid but minor streets are higgledy piggledy.

  9. I think it’s possible to do higgledy piggledy without cul-de-sacs. If you want to slow cars down and make streets more people friendly then they don’t need to be “dead end”. They just need to be narrower.

  10. Certainly one of the things I’ve noticed as a cycle commuter, first in Auckland then in Wellington, is that drivers seem more aware of me in Wellington. I think the extra care they need to take on narrow streets is a big part of that.

  11. Its quite plain looking at the new subdivision patterns that public transport planning isnt even on the radar. Whats important is that they maintain a grid at 800m. If developers what meandering roads within that grid, thats up to them as it wouldn’t be impacting on PT design. Manukau has to be the worst example of urban development in Auckland. Every one is channeled onto the same arterials to get out of the place and look at the gridlock that ensues and traffic engineers think they have it right? Duh! Grids are good at reducing congestion as per Obi’s post.

  12. Cam, I guess the idea is that all these routes run at 10 minute frequencies all day long (higher at peak). In between them could be routes that run at lower frequencies. Certainly a Richmond Rd route would be necessary, as would other routes to fill in the larger gaps.

    I guess the point of the above diagram is to show how much of the isthmus is covered by only 16 routes.

  13. Well, ironically, I once got totally stuck in the Greenhithe areas when I tried to cycle through there. Yep, not only was it a cul-de-sac, I also wasn’t in a car, so couln’t use the stupid motorway, and had to slog my way back uphill for a horrible distance when I realised there was no way to get onwards. The fact that the area is very hilly and this harder to connect didn’t really appease me.

    Alleys can be done well. Just don’t make them 2.5m wide! They need to be 6-8m wide, if for no other reason than in that case you could still change them into through roads later on if you really needed to. Oh, and for all that I like the idea of cul-de-sacs combined with walk & cycle alleyways, they still wouldn’t allow buses…

    I need to check whether they followed my submissions regarding cul-de-sacs in the new subdivision standard that just came out. They were on the right way in the draft, but not really prescriptive enough against them yet.

  14. Oh, and while I agree with most of the comments Jarbury made above, I think the very first diagram is actually very misleading. The left suburb (the “bad bad cul-de-sac!” one) is actually also separated by the barrier effects of what looks like a motorway, a river and another potential high-volume highway-tyope road (hard to tell). In the right hand example, none of these three are present, and it looks like a higher density area (in the left-hand side map, some areas seem to be empty in fact, noting the lack of streets coming anywhere close at all, walkable or not).

    So of course the right-hand image area is more walkable! To argue that this is due to the absence of cul-de-sacs is misleading – it’s a number of other things too!

  15. Max, the cul-de-sac, arterial feeder and motorway are all key features of the ‘heirarchy of roads’ type of road layout, so I think the diagram is quite apt. The cul-de-sac is just the lowest rung on the non-grid street layout, such layouts are designed to take vehicular traffic from local culs-de-sac to arterial roads and then on to motorways.

    Indeed there are large areas of open land along the freeway and arterial roads in the first diagram, such barrier effects are a common feature of that type of street pattern and a major part of the problem.

    You make a valid point about the river, but note in the ‘heirarchy of roads’ street pattern on the left that there are only two crossings of the river, one by what looks to be a freeway interchange and the other by arterial road… a small number of high capacity road bridges is likewise characteristic of that type of street pattern. If you had the same river in amongst a fine grained grid layout you would most likely see a larger number of smaller local crossings that would make the river a lot easier to cross on foot or by bicycle. You see that effect in Melbourne, in the older ‘walkable’ suburbs there is a local road bridge across the Yarra river every 8-900m, while in the newer ‘driving only’ suburbs the river crossings are major arterials or freeways spaced every 4-5km.

  16. Fair enough – though I feel the hierarchy of roads actually makes a lot of sense. It has just been driven to extremes.

  17. I agree it makes sense… if you are a traffic engineer whose only concern is to move people by car as quickly as possible while minimising traffic on residential streets.

    However if you are trying to do anything that doesn’t involve driving a car to your nearest motorway it is the complete opposite of sensible.

  18. Nick R, that is a pretty superficial and even agressive remark. Don’t assume that traffic engineers are blinkered folks – some of us may be, but that doesn’t mean we are the public’s enemies as you seem to assume.

    In the whole conflict between “place” and “movement”, you should better remember that movement is not an evil. I believe that a road hierarchy and a grid network are far from incompatible.

  19. Surely the best way to create a road hierarchy is simply through the width of the street, rather than the shape of it. I agree with Max that a grid and a hierarchy are compatible, and having a hierarchy is probably a good thing. Your arterials can be quite wide (say two general lanes plus two bus lanes plus a median) while your local roads are narrow enough to ensure that drivers have to go slowly (generally I have found the most effective way of doing that is to make the width narrow enough that only one car can pass through if cars are parked on both sides of the road.)

  20. Max, I’m not saying traffic engineers are ignorant morons or anything, I’m just pointing out that the mandate of a traffic engineer is to engineer traffic. The problem lies in the fact that historically, and to a lesser extent currently, decisions on street layouts were made as a ‘technical issue’ related to promoting traffic flow and keeping speeding cars out of residential streets. This isn’t a bad thing in itself, but it has resulted in some adverse implications for urban form and accessibility. Luckily these days it seems that the planners and engineers working on street layouts have a wider brief that just optimizing traffic flow.
    I agree that having a hierarchy of roads can be very usefull, indeed most successful grids are comprised of a mix of smaller local streets, a wider grid of arterial plus connections to large highways and freeways. It’s very car focussed application of culs de sac and curvilinear feeders I think we need to move on from, although I can see ways in which culs de sac and curved roads could be the basis of an easily walkable (and drivable) street topography.

  21. Thanks for the praise, Jeremy – though I’d rather not reopen old “feuds” with Riggles! (who never posts here – unless he has a different nickname).

    Nick R – I found it interesting to read recently that “Flatbush V2.0” in Manukau is now supposed to go away from curved streets, back to straight ones. I am not a fan of straight streets – they are boring (New York-type grids for me are the epitome of soulless), and ignore the landscape contours, and encourage speeding.

    I think the main issue is that combining a grid network with a “strong” hierarchy network well is that it requires much more grade separation to overcome the barrier effects, which is rather expensive, and can be difficult to do well, especially for the pedestrian and cyclist groups.

  22. I don’t think grids have to be comprised of straight streets necessarily, I think the key characteristic is the interconnectedness of the lower levels of the hieirarchy. I agree very regular grids are boring. One thing I like about inner Melbourne’s grid is that the regular arterial grid makes driving and tram travel easy, while the irregular local streets and lanes inside the grid add ‘drama’ to the urban environment and make walking and cycling interesting. I walk for transport a lot, but I’d certainly prefer to do it via local streets than down the side of a major arterial, even if the latter was more direct.

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