A while ago Kent and I made a proposal for a tree lined boulevard in Auckland. Curiously, the biggest theme of the comments section was about the perceived lack of value that trees bring to the street corridor. It seems that most people consider street trees to be at best, decoration, and at worst a waste of time and money, dangerous even. Indeed I have seen road design handbooks whose only mention of trees was to outline all their problems in the section of ‘non-frangible fixed hazards’ (in traffic engineering terms, frangible means something that will break off when you drive into it, rather than stay solid and crumple your car around it).

I wanted to use this post to outline some of the reasons why trees are beneficial for our city streets. Not just beneficial from appearance or character, but beneficial in the sense of making the street work better in terms of its transportation and land use.

A word of clarification up front however. I am  talking about trees on city streets, particularly urban inner city streets with speed limits of 50km/h or less and ‘stuff’ around that people actually do there (like live, work or play, and not just move). Trees on rural highways and back roads with nothing going on but through traffic are a different story.

So my point is street trees are not merely decoration and can be included in street design specifically for several practical, technical reasons. In no particular order, these are as follows:

 An outer row of trees can provide physical separation between the traffic lanes and the footpath, cycleway and retail frontages.

Your friendly local highway designer will tell you that trees are a fixed hazard, according to the Austroads design manual at least, and that a high speed highway needs a wide exclusion zone either side so that speeding drivers who run off the road can careen onwards without hitting anything. While this is a very appropriate safety treatment for a state highway out the back of Waipukarau, it is completely inappropriate for a city centre arterial.

For a start we don’t want a high speed highways through our inner city. That what the motorways are for and we don’t want cars and trucks to speed through dense people-focussed places at street level. Rather we want move plenty of vehicles in an efficient manner while keeping to a reasonable speed limit for a city arterial. Consistent reliable travel times and total throughput are far more important that high speed alone.

Secondly, we certainly don’t want vehicles running off the traffic lanes at high speed. In an urban context this means running over whoever happens to be walking on the footpath at the time and smashing into the front window of a shop or office building! We want to keep traffic in the traffic lanes.

Thirdly, we don’t want to waste scarce city land on wide empty shoulders for our main roads: we can’t afford the land and don’t want the severance it creates. Nobody wants downtown to look like Albany (and in fact I’m not sure if anyone especially wants Albany to look like Albany either!), and we shouldn’t really be spending good money to create empty verges in town centres.

So indeed, one very practical reason for street trees is to keep traffic in the traffic lanes and out of the footpath and shops. We could use bollards or Jersey barriers for separation, or wide swathes of land, or we could use trees.

Better to hit a tree than a primary school?
Better to hit a tree than a primary school?

Trees provide a physical barrier to prevent vehicles being parked across the kerb.

This physically stops people from parking vehicles on the footpath, or in medians or other non-traffic spaces. Again we could install bollards all the way along the edge of the road, but trees can do the same job too.

A physical barrier to stop people parking on the footpath.
A physical barrier to stop people parking on the footpath.

Street trees provide shade to the footpath and cycleway.

Shade would not be of concern to someone designing a rural highway, where they don’t want nor expect any pedestrians or anyone else not in a vehicle for that matter. But if we are talking inner city streets such things must be considered. People walk, cycle, wait, linger and loiter on city streets and a little shade goes a long way in the summertime. Indeed in Melbourne they have started a program of street tree planting to mitigate the increasing number of heatwaves, and shading the tarmac is a good way to reduce the urban heat island effect and manage stormwater. Trees are a handy component of a complete street that has many simultaneous uses, and users.

Well placed trees can shade the footpath to provide a comfortable walking environment.
Well placed trees can shade the footpath to provide a comfortable walking environment.

Trees can visually screen heavy traffic from the footpath and adjacent buildings.

…and to a lesser degree provide some containment of noise and fumes. Once again this is a very practical concern if we are discussing inner city corridors, particularly those that are ripe for commercial development. Heavy traffic is a necessary evil on city arterials, generally speaking, but it’s impact can be mitigated through design elements such as this.

Trees provide a perceptual barrier that visually narrows the the carriageway.

Perhaps most importantly, a rows of trees can be used to visually narrow a simple two way street or split the opposing directions of a multi-lane arterial with a physical and perceptual barrier. In the case of a multiway boulevard a row of trees can be used separate the local and through lanes from each other.

Overseas, using trees to separate lanes is an intentional feature designed to break up the wide roadway into discrete sections, each section being relatively narrow. The purpose of this is to perceptually narrow and contain each piece of road to no more than two  narrow lanes to remove the visual cues that encourage speeding. In effect the trees act like side walls, narrowing each part of the road and forcing drivers to maintain limited speeds. Without them, any multi-lane street would present you with a wide, straight, flat roadway many lanes wide, all if which tell your subconscious mind that you are able to put your foot down. Of course the highway handbooks define this as the problem of ‘shy lines’, assuming lanes should be as fast and wide as possible and describing anything less in terms of reduced capacity. But this capacity reduction comes from lower speeds, which is precisely the thing we are after on streets, if not highways. Furthermore the shy line effect is almost negligible on 50km/h arterials, it’s really a highway thing.

The literature is quite clear on this topic: people drive fast on big wide roads, and creating a visually constrained roadway is the best means to keep then to the speed limit (ever been infuriated driving on the highway behind someone who is slow on the single lane sections, but who speeds up second they get to the passing lane? That’s them simply responding to the form of the road: going slow and cautious on the narrow winding bits, and driving faster when it goes wide and straight. Its a simple, instinctive perceptual response…)

So we prefer to see this ‘problem’ as part of the solution. Here the trees become a perceptual traffic safety device for reducing speeding and keeping traffic moving efficiently at the proper speed limit.

Octavia Boulevard, SF. Rows of trees turn an eight lane monster into groups of two lane streets.
Octavia Boulevard, SF. Rows of trees turn an eight lane monster into groups of two lane streets.

But what about simple beautification? 

Finally however, what if the trees were just ‘useless’ beautification? And what of the potential for ‘useless’ good quality paving, street furniture, artworks even? Is it a waste of time to make things attractive? Should we spend money on a nice looking and pleasant urban environment?

In the case of the city centre or other town centres, absolutely we should. Recall what we are talking about here are on the whole prime commercial redevelopment sites, where we should aim to maximise both the sale value of the land release, and the intensity and value of the subsequent developments. Simple ‘beautification’ would reap big dividends on the final form of development in the corridor. That’s not to say street trees are all you need to create a blue chip commercial precinct bringing in rates and business investment, but they definitely contribute. Beautification is a practical function when it comes to catalysing redevelopment.

An "un-beautified" road. What sort of development would this attract? Hat tip stroadtoboulevard.tumblr.com
An “un-beautified” road. What sort of development would this attract? Hat tip stroadtoboulevard.tumblr.com

So there we are, several reasons why street trees should be considered as technical components of street design with specific transport outcomes. For a wider discussion of all the benefits of street trees, transport and otherwise, check out the 22 Benefits of Urban Street Trees by Dan Burden.

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  1. Portland Oregon and some other cities in North America have studied the economic value of trees fairly extensively. I recall findings such as “A good street tree adds $10k to the selling price of a house” and “a leafy street increases house values 15% compared to a street without trees.” One cool project that’s nice is the National Tree Benefit Calculator: http://www.treebenefits.com/ this is a USA tool, but we could probably compare values some of the Leafy vs non-leafy streets in Westmere/Grey Lynn which are similar in many respects except the trees to gauge actual value.
    In an urban setting, the ‘green buffering effect’ can’t be overrated – arterial roads (with several lanes, cycling, buses, streetlights) are really not good places to put housing next to – without trees, this can become unbearable. Yet we expect apartments and other high-intensity uses to be attracted to transit corridors.

    Trees also provide habitats/biodiversity and have a role in stormwater managment. Oh, and we’re now doing subdivisions with fruit trees. Timber production might be another benefit.

  2. I haven’t seen numbers for a while but research has shown that street trees increase property values on residential streets.

    As for frangible, my friend’s father was an oral surgeon and he said the worst thing you can hit is a tree.

      1. Well, yes, it stands to reason. But with your mouth in a car, too. He’s pieced together plenty of faces, not just mouths. And has the film to show it. We loved that.

    1. Just look at Franklin Road – beautiful tree lined road with property values to match.

      There are a parallel roads such as College Hill Road that has been more optimised for traffic flows that is comparative awful without the trees shading the footh-paths.

      I would love to buy a house in Franklin .. and I am obviously not alone in that.

  3. I think trees are great and essential and Auckland needs more.

    But, BUT,

    They also limit you. If anyone read my thing mentioned on Sunday about the Vancouver cycle lanes, a big roadblock to doing some simple fixes are trees – there is space for a cyclelane, if not for the trees. I don’t mean to say cycle lanes (or anything else) > trees, more that, we need to be damn sure we’re getting it 100% right because once those trees grow, they aren’t going anywhere. My concern is that AT is still ‘learning’ about how to do some of this infrastructure and trees will mean we get stuck with it indefinitely.

    I’m a big fan of the trees outside the ferry terminal. Well, actually I hate them, because they cut the sidewalk down and make it too tight to cycle on. But they are movable and I’ve seen them all over the place. It brings flexibility while lessening the harshness. It’s not an answer for 80% of situations perhaps, but another tool.

  4. Bear in mind that all these mentions of benefits are from the US or Oz where roads were a decent width to start with. Very few of our arterial routes fall into the same basket, most being only 1 lane each way and parking. Trees on the roves of buildings – now that is another matter and would be a good idea to lower temperatures, deal with storm water etc. The trouble in Auckland is that someone will get a ‘wonderful idea’ and then totally stuff up because they don’t think it through properly – just like the ever increasing numbers of speed bump installations which really (be honest) achieve nothing other than annoying vehicle drivers. Be careful or some pratt will remove lanes just to plant trees and think they have done everyone a favour. If you look at the pix of the US streets the wide real estate was already there – we don’t have that to begin with.

    1. ‘Be careful or some pratt will remove lanes just to plant trees and think they have done everyone a favour.’


      the tears of the traffic engineer… won’t somebody just think of the tarmac….

    2. “Be careful or some pratt will remove lanes just to plant trees and think they have done everyone a favour.” – Is “pratt” some slang term for enlightened urban planner that I have never heard of?

      If so, we need many more pratts in this city. Even better if they remove lanes so that people on bicycles have more dedicated space.

    3. Many arterials are neither one lane or two lane – they are one and a half lanes wide with no road marking. They aren’t wide enough for two cars, so they are in effect a very wide single lane road. In my local area, I’m thinking Mt Albert Road and Mt Eden Road – although they have bus lanes on some parts of them.

      At the moment we are all paying for that “half lane” that isn’t actually doing anything useful at the moment, except sometimes providing car parking. I would far prefer it be used for trees or busses or bikes, or all of these.

      And on a slight aside: I visited Portland OR a few years ago, and even the crap suburbs look posh when they have street trees. Downside: leaf clean up in Autumn must be diabolical.

    4. Sorry Ricardo, don’t agree at all. Basically every street in Auckland has either a grassed verge or a section of the footpath width by the kerb that all the poles, sign posts, bins, post boxes etc sit in. The verge on some main arterials are huge. Trees can be located in these without going into carriage way, however I’m not really worried if we did. Removing some of the parking for trees would be just fine by me.

    5. Not all. London’s trees have been valued at 4bn pounds. Ie if they were all removed, 4bn would theoretically need to be spent on stormwater infrastructure, traffic calming, mental health services etc etc.

  5. “…ever been infuriated driving on the highway behind someone who is slow on the single lane sections, but who speeds up second they get to the passing lane? That’s them simply responding to the form of the road: going slow and cautious on the narrow winding bits, and driving faster when it goes wide and straight. Its a simple, instinctive perceptual response….” Seriously? They are simply arrogant drivers, they know they are holding people up – more of the righteous ‘I’m doing the limit or below, so to hell with everyone else’ attitude. That is why they speed up when passing lanes occur to help hold back ‘the speedsters’ and any one else they think is doing wrong. Plain arrogance. That is why the Police ticket them.

    1. No I don’t thinks it’s arrogance – just lack of awareness. The road itself gives the cue which causes drivers to adapt subconsciously.

      Try driving at the speed limit along the eastern section of Quay Street past the port, and on Tamaki Drive, and see the tailgaters close up on you. It’s a natural instinct to speed up when the road suddenly widens, and it takes a conscious act of the will to keep to the same speed. Which illustrates the importance of good street design to build in the cues to drivers – known in some parts as “self-explaining roads.

      1. I ride tamaki drive every day. I don’t tailgate because i ride a motorbike so i just go straight past everybody but when I’m in front keeping 50kmh is really hard. Time they cut it down to two lanes and busway either side.

    2. Been there, seen that too. Coming from oversees, it took me a while to get used to driving 100km/h on a 2-lane road. So I can sympathize with speeding up a bit.

      But yes some people really overdo it. Accelerating from 70 to 100 is not something you do unconsciously. And it’s not like every single stretch of highway without a passing lane is “narrow and windy”

      On the other hand, if you’re a bit back in the queue, what you usually see is not the slow car speeding up, it’s the people in the queue in front of you speeding up when they overtake the slow car.

  6. Hmmmmmm, more trees would be nice. Especially colourful ones like jacaranda trees and kokowhai. Might need to make sure they don’t drop too much leaves though. Mushy leaves on concrete is bloody slippery to walk on.

  7. The second group of photos is an absolute plague at the moment. And its not just the Strand or industrial areas but commonly around the nice leafy suburbs where you will find every man/woman and their contractor (renovations) thinking its sweet as to block the footpath. Meaning either I go up their drive if possible to go around their machine or risk it out on the road. Too bad if you have little kids and or a pram, its the road or nothing.

    This is one area or stationary vehicle offences I would dearly love to see AT hammer!.

  8. What a shame that most people “know the cost of everything and the value of nothing”.

    The most compelling reason to plant trees is to beautify our cities, all other benefits are secondary.

  9. Meanwhile in Palmerston North, the Council go and chop down all the lovely trees down Broadway. Because too many birds were in them in the evening, shitting on the parked cars + footpath underneath (I presume removing the parks and water blasting every now and then wasn’t a solution to be entertained).

    Now the birds are just more concentrated on the trees in the square, I presume, and Broadway looks like a wasteland without the trees.

        1. That is just design and maintenance though, it just means we need to invest more than a token amount in our walking infrastructure.

    1. Unless there’s a hazard I prefer them not to fix it. There’s a few where I live cracking into the footpath, and it makes for a beautiful and somewhat poetic sight to see the roots breaking up concrete and making chaotic patterns.

      Certainly better than miles of solid grey concrete, anyway.

      1. There is nothing poetic about tree roots cracking the footpath when you are trying to jog along them with the dog at dusk. I need to avoid certain local streets in poor lighting (e.g. south side of Campbell Rd).

    2. It’s true though. Lots of old people walking around. You’d be surprised how many trip over small bumps on the footpath, break their hip and spend months in hospital. With ACC we all pay the costs, so preventing accidents is very cost effective. Trees lift up pavements and cause the uneven surfaces, so they are the cause of the problem in narrow carriageways where we can’t plant the trees far enough away from the footpath.

      Personally I think there are many trees that don’t beautify the place and just cause too much trouble. I like Plane trees but they are so costly to maintain.

    3. that is purely a design fault. Planting trees without considering their root structure and eventual size.
      Nowadays there are ways to negate that, with root cells that allow room under pavement etc. for the roots to grow.

  10. It’s instructive, surely, that people refer to ‘leafy’ suburbs when they mean high-value ones. Leafy to me means street trees. The roads in otherwise ordinary suburbs that have trees creating a nice avenue – like Franklin Rd, like Calgary St in Sandringham – have much more charm. I’m happy to have them for mental health and aesthetic reasons, but it’s interesting to read about the traffic engineer benefits too.

  11. It’s been a real pleasure to watch the trees grow over the years in various Auckland suburbs. They’ve transformed Mangere, for example, from a barren place into something established and calm.

    However the major constraint against trees in Auckland has been the fact that people travelling at speed drive into them, at great harm to them and their passengers. At various times they’ve been removed from roads for this reason. This will remain so until we get self-driving cars, and so for the meantime they need to be appropriate to the roads they’re on and the speeds that people are travelling. On quiet roads, these might be larger trees, on busier and faster arterials they’ll be smaller trees with more pliable trunks. Choosing trees that limit the amount of foliage they drop is also important, for maintenance considerations. But all of these are solveable problems.

    1. Good points George, but that is simply a design question. Over the years we have changed the design of our power poles, lamp posts, signs etc so that they are appropriate to the type of road and so they don’t especially damage vehicles or people that strike them (certainly didn’t need to wait for driverless cars for that). Do the same with trees.

  12. Great contribution. Trees add a lot of value to the urban environment. In Nairobi however, motorists do not respect them, they drive over them and outdoor advertisers cut them to enhance their visibility on the main road corridors

  13. I do training on sustainable urban transport projects here in Sarawak. It is almost imossible to send messages that tree line street has good value. We did have great tree line street in this little city of Kuching in Sarawak, Borneo Island. Guess what. With a craving for expressway and flyover, the tree line street along the airport road were destroyed creating heat island. A problem with local authorities here in Malaysia generally is the reluctance to plant tropical trees that can provide shades claiming these invite birds, swallows and swifts. Difficult to maintain. Another jaw drawing response from a mayor in Kuching is that big trees are dangerous.

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