A great article on transport modelling did the rounds over the past couple of weeks, raising questions about how much reliance we put on these algorithms in our transport decision-making process. We have criticised transport modelling on many occasions over the years, especially in relation to the City Rail Link business case – where inadequate modelling probably led to the project being delayed for a number of years.

The recent article on Vice.com has a higher level criticism than many of the technical flaws we’ve identified. It does this by asking whether transport modelling is behind many of the undoubtedly terrible transport decisions that have been made over the years. But before it gets to that issue, the article helpfully provides a bit of a “Modelling 101” guide:

Travel demand models come in different shapes and sizes. They can cover entire metro regions spanning across state lines or tackle a small stretch of a suburban roadway. And they have gotten more complicated over time. But they are rooted in what’s called the Four Step process, a rough approximation of how humans make decisions about getting from A to B. At the end, the model spits out numbers estimating how many trips there will be along certain routes.

As befits its name, the model goes through four steps in order to arrive at that number. First, it generates a kind of algorithmic map based on expected land use patterns (businesses will generate more trips than homes) and socio-economic factors (for example, high rates of employment will generate more trips than lower ones). Then it will estimate where people will generally be coming from and going to. The third step is to guess how they will get there, and the fourth is to then plot their actual routes, based mostly on travel time. The end result is a number of how many trips there will be in the project area and how long it will take to get around. Engineers and planners will then add a new highway, transit line, bridge, or other travel infrastructure to the model and see how things change. Or they will change the numbers in the first step to account for expected population or employment growth into the future. Often, these numbers are then used by policymakers to justify a given project, whether it’s a highway expansion or a light rail line.

Major transport plans and policies in Auckland have leaned very heavily on transport demand models. For some pieces of work it seems like the models have provided some useful insights – such as in ATAP where modelling indicated that Auckland simply can’t build its way out of congestion, instead needing to manage travel demand through initiatives like road pricing. But for other pieces of work models seem incredibly unsuitable – like the ‘Access for Everyone’ city centre transport strategy, where Auckland Transport’s implementation work is relying on transport modelling that’s incapable of understanding how making driving less attractive will result in fewer people driving. This is just setting the process up for failure (either inadvertently or deliberately, one can never tell with Auckland Transport) and raises the issue of why the transport profession has abdicated responsibility to transport models so much. The Vice magazine article picks this up further:

Either way, nearly everyone agreed the biggest question is not whether the models can yield better results, but why we rely on them so much in the first place. At the heart of the matter is not a debate about TDMs or modeling in general, but the process for how we decide what our cities should look like.

TDMs, its critics say, are emblematic of an antiquated planning process that optimizes for traffic flow and promotes highway construction. It’s well past time, they argue, to think differently about what we’re building for.

“This is the fundamental problem with transportation modeling and the way it’s used,” said Beth Osborne, director of the non-profit Transportation for America. “We think the model is giving us the answer. That’s irresponsible. Nothing gives us the answer. We give us the answer.”

A recurrent theme over the past 50 or so years in land-use and transport planning has been this idea that key decisions about where to grow our cities or what projects to build are simply responding to the preferences the public has, rather than shaping the very options people have available to them. For transport, models make travel demand seem like a completely external force that must be predicted and then provided for. This approach is central to the thinking that has got us into the congested, unsafe, unsustainable mess we find ourselves in. Most transport models (and most traffic engineers to be honest) think of travel demand like water and roads like stormwater pipes. In reality though, travel decision-making is way more complicated.

In the models, any trip made today will be made perpetually into the future no matter how much worse traffic gets.

Experts refer to this as “fixed travel demand,” which is essentially an oxymoron, because travel demand is almost by definition not fixed. We are always deciding whether a trip is worth taking before we take it. One of the major factors in that decision-making process is how long the trip will take. TDMs work the exact opposite way by assuming that if people want to go somewhere they will. Only then will they calculate how long it will take.

For this reason, some urban planners derisively refer to this approach as “the lemming theory of demand,” said Joe Cortright, an urban economist for the consulting firm Impresa and contributor to the website City Observatory, because it assumes people will keep plowing onto highways no matter how bad congestion gets.

“It’s not so much about the measurement being wrong, it’s that the whole underlying thesis is wrong,” said University of Connecticut professor Norman Garrick. “You’re not thinking about how people behave and how they’re using the system. You’re just saying this is how it happened in the past [and] this is how it will happen in the future, even though you’re injecting this big change into the system.”

While most good strategic models don’t have completely fixed demand – a new public transport project can affect the mode choice people make for example – many of the model’s fundamentals like daily trip numbers are fixed. Furthermore, from what I understand some of the more detailed models (like what Auckland Transport uses in the city centre) do have completely fixed demand, meaning that they literally aren’t able to replicate the real behaviour changes people make in response to changes on the street network.

Models also tend to under-estimate ‘induced demand’ – which probably explains why our motorway remain congested (at least before Covid lockdowns!) despite countless past projects that were meant to fix that congestion.

This phenomenon is called induced demand, and it is not merely a thought exercise. It is precisely what has happened in nearly every case where cities build new highways or expand old ones.

“Recent experience on expressways in large U.S. cities suggests that traffic congestion is here forever,” wrote economist Anthony Downs in his 1962 paper The Law of Peak-Hour Expressway Congestion. “Apparently, no matter how many new superroads are built connecting outlying areas with the downtown business district, auto-driving commuters still move to a crawl during the morning and evening rush hours.”

Experts have known about induced demand for generations, yet we keep adding more highways in the Sisyphean task of attempting to build our way out of rush hour traffic. To fully appreciate the absurdity of this quest, look no further than the $2.8 billion freeway project in Katy, Texas that was supposed to reduce commute times along the expanded 23-lane freeway, the widest in the world. All too predictably, congestion only increased, and commute times are longer still.

2011 paper called “The Fundamental Law of Road Congestion” concluded “increased provision of roads or public transit is unlikely to relieve congestion” because every time new lane-miles are added, trip miles driven increase proportionately. The more highways and roads we build, the more we drive. (The flip side is also true: in the rare cases when highways are temporarily out of commission, such as the case with the Alaskan Way Viaduct in Seattle, traffic doesn’t get much worse.) And TDMs have been totally ignorant of it.

Does this mean transport modelling is a complete waste of time? Should we stop using models altogether? Not necessarily.

Jarrett Walker, commenting on the same article in a recent post, highlights that modelling can be useful when used properly – but it should never be the decision-maker and it should never be seen as simulating exactly what will happen. Models make far too many assumptions about the future for that to ever be true. As Jarrett illustrates, many of these assumptions are based on what’s happened in the past – which makes sense from some perspective as that’s the evidence we have. But from another perspective, especially if you’re deliberately trying to achieve change, these assumptions might be wildly wrong.

The best modeling is not nearly as dumb as the examples Gordon highlights.  But the problem of all modeling is that to show the effects of a proposed action, you have to assume that everything else in the background will remain constant, or at least will continue changing only along predictable paths.

When the modeling process considers many possible futures, the one that is most like the past is called the conservative assumption, as if that means “this is the safest thing to assume.”  This assumption seems calm and rational, attracting many people who would never call themselves conservative politically.  But fact, assuming that the future will be like the past can be crazy if the trajectory defined by the past is unsustainable — environmentally, financially, or morally.  “Unsustainable” means that it is going to change, and in that case, the “conservative” assumption is really the “self-delusion” assumption.

Transport modeling can’t be thrown out, but it never tells us what to do.  It is a basic logical fallacy to say that “the modeling shows we must do x.”  All modeling insights are if-then statements.  A full version of this statement, which I would like to see at the beginning of every modeling-drive transportation study, is:  “This report shows that if the future matches our assumptions, then you can expect this outcome.  But the future may not be like that.  In fact, maybe it shouldn’t be like that.  So what really happens is up to you.”

So perhaps modelling isn’t complete junk science. But always be enormously sceptical when people appear to be abdicating responsibility for a decision to modelling. And always question whether the modelling is simply guiding us to repeat the mistakes of the past.

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57 comments

  1. As alluded to about CRL; most of the limitations and caveats that apply to transport modelling also apply to cost benefit analysis. Partly because of how reliant the latter is on the former. Plus cost benefit analysis introduces another layer of assumptions that can be misleading.

  2. All the PT models are crap in the face of this pandemic.
    And will be for years until there’s a fully implemented vaccine AND the models can cope with reinfection surges.

    1. No models would stack up in a pandemic, they are not really intended to.
      In the not too distant future we will have a cure for Covid, it will disappear, or we will learn to live with it. I doubt we will even be talking about it in 5 years time (which is not a long time in terms of transport planning). I don’t think Covid is the “new normal” at all.

      1. What do you mean learn to live with it though jimbojones.? That would mean probably being more hygienic and looking after ourselves and our families at the slight sign of a cold. But also keeping distance.

        To me that means the new normal for Covid is a lot about cycling and in all weather so it has to be really safe.

    2. We had much worse plagues & pandemics, and humanity hasn’t abandoned living in dense cities. So PT remains relevant. But comments like the above will hurt necessary PT investment for many years “Well, it didn’t exactly help during the Pandemic, so why should we build this [busway/light rail/whatever] – roads are a safer bet!”

      1. For most of human history, cities have been death traps. Experiencing pandemics and other infectious diseases (the focus of the so-called old public health… although that term seems a bit odd these days) in that context is very different to a world where people have had decades of getting used to the idea that city growth isn’t driven wholly by migration.

        In terms of Covid’s reinforcing automobility, I would focus more on the utter autocentric approach to public health. “Stay in your vehicle”. Okay, cool. What if you don’t have a vehicle? Need a test? Ah, you can’t come into the building if you have respiratory symptoms… After all, you’re not meant to travel long distances… we’ve spent months being told to focus on precisely the scale in which active modes fit in.

  3. I think it was the economist Ronald Coase who said “If you torture the data long enough it will confess”. He must have understood how people build models.

    1. I would like to model the percentage of times that modeling happens to result in the exact outcome the sponsor wanted. But as the sponsor I already know the outcome so not much point.

  4. Not sure what conclusion to draw from this. OK, so TDMs need to be treated with caution and should not be relied on to provide watertight conclusions to the outcome of transport investment decisions. They should inform decision making but not be the decision maker.

    So how SHOULD we make such decisions? The obvious answer is that if TDMS are de-emphasised they’ll become more and more politicised, and less rooted in logic and reality. That may feel good and produce inspired and creative decisions if you have political decision makers who are progressive and in touch with the needs of the environment and the economically disenfranchised parts of the community. But if you have decision makers who are beholden to a narrow elite and are out of touch with real community needs and the environment then the outcomes could be absolutely disastrous.

    Frankly, when it comes to transport decision making, like Covid, I place more trust in the “experts” than I do politicians. Sure, politicians will always (rightly) have the final say, but I’d hate to go back to the bad old days when transport decisions were informed primarily by political or sectional self-interest, rather than what’s really best for Auckland.

    So what’s the answer? I really don’t know, but let’s not toss out the tools that at least try to provide objective advice to the public and decision makers.

    1. Out of interest, would you prefer the whole country to be run by “experts” rather than by politicians / democracy?

      1. Not at all, and that’s exactly the dilemma we face. How much weight do you give to transport modeling, which we know to be flawed, and how much weight do you give to politicians who (we know) are equally flawed? Ultimately the decisions will be taken by partisan politicians who hold the purse strings, but will they make better decisions by using TDMs which may or may not be accurate? Or should they trust their gut?

    2. “when it comes to transport decision making, like Covid, I place more trust in the “experts” than I do politicians.”

      I think we’re allowed to look at the big picture and say, “Whatever got us into this state isn’t the tool or the process we need now. Both the models and the politics, perhaps, have failed us.”

      1. Don’t disagree with that, Heidi, but I can’t think of any alternative model that doesn’t ultimately give the power to politicians. Which is as it should be, given they’re our elected representatives but just how much power should they have? Given the sorry, pork-barrel, regionally disconnected and car-focused history of political decision making on transport issues pre-AT, at least the “experts” can (hopefully) be more dispassionate. Personally, I think that the current situation, where Council sets its expectations and strategy, and reviews proposed annual spending in detail once a year before signing off AT’s annual programme, is just about right. Then the local boards get to be involved in detailed discussions about individual projects before they commence, and changes are often made at that stage.

        Some people have proposed “co-design” between the politicians and AT, but all I can think of when I hear that is interminable discussions about whether bus lanes are required where perceived political survival mode kicks in and politicians take fright. Sometimes the transport modelling around bus lanes is the only argument that can (barely) persuade politicians not to rock the boat. And even then there’s always an outlier who takes a Trumpian anti-logic position and tries to fire up the community with stupid, illogical and sometimes downright dangerous arguments.

        I agree the present model has many flaws, but what alternative model can be found that would work better? Let’s not tear down the status quo until we know what we’re walking into.

        1. There are definitely better models available. NZ is using archaic stuff. But the two changes are needed side by side. Better modelling and more thinking about what role the model can give.

          Experts can still advise on transport using evidence and science without it being modelling-based. The models can then be brought in to answer specific questions.

    3. The problem is that the models are already political. The way pedestrian, cyclist, PT user and different classes of driver are costed is an inherently political decision that’s been baked into the system right at the beginning. When you say you want ‘experts’ to make the decisions instead of politicians, you’re really trying to move certain things out of the realm of acceptable politics.

      There are different levels of this, of course. Central bank independence is almost entirely a political matter, while climate modelling has very few political assumptions, at least when you’re just doing the modelling. This probably sits somewhere in the middle, but it’s as important to be open about the political assumptions that have been made as about the physical assumptions

    1. This is the kind of decision that gets made when TDMs are relegated to second place behind narrow sectional interests (in this case the re-election of the government and the need to appease the car lobby).

      1. No, I think it’s the kind of decision that can only be made when the modellers and ‘transport experts’ are ignoring several decades’ worth of evidence, and myths like this are fed to the politicians.

        This myth can be traced right down into the actual benefits and disbenefits valued in the multi criteria analyses and business cases.

        The effect on health, safety and the environment of widening highways due to the engineers at NZTA touting such debunked nonsense mean they are not meeting the code of ethics for their profession.

        1. Fair point, but if the “experts” aren’t there to give the politicians advice, where will they get their advice from? Perhaps from the internet?

          Yes, there are many unreconstructed engineers working for various transport agencies, but short of firing the lot of them (which isn’t going to happen, realistically) they’ll still be there. There are also many excellent transport engineers who do understand what needs to be done. But what I think we need is BETTER transport models, rather than dismissing them as “junk science” and leaving a gaping void in the information used in decision process. Because we already have clear evidence of what happens when the politicians are the only hands on the tiller.

        2. The experts need to be there to give the advice, absolutely. But the advice needs to be best practice.

          And right now, that means pulling back from models that haven’t served us well. Replacing them for now with techniques such as tactical urbanism, reallocating road space, implementing LTN’s – even if the old model would’ve said no, and monitoring extremely well, in order to create better models.

          It means starting with a different end point. Instead of “sprawl will happen, roads will be built, people will be located here – now where will the trips be, and how do we accommodate them?” they need to ask “intensification will happen, roads will be reallocated, vkt will drop – now, where will the trips be, and how do we accommodate them?”

          With this approach, they will find the sort of investment in infrastructure required is Very Different from what they are investing in, and the whole NLTP will need to be turned on its head. As will other infrastructure funding.

          Will they be able to use the same traffic models for this? No, they’ll find the current ones unusable. But the staff who have been keeping abreast of better models will be able to step up to resolve the situation.

        3. “what I think we need is BETTER transport models, rather than dismissing them as “junk science” and leaving a gaping void in the information used in decision process.” +1

          My guess is that there is a lot of inertia in organisations like AT and consultancies holding back adoption of better modelling techniques.

        4. Great discussion here.
          Both @Heidi and @DavidByrne have good points.

          The point about politicians needing models and other ‘hard’ tools is pertinent because it allows them to not take a stand on something they want to support. This way they can just defer to the magical model that says what must be. This is actually also helpful for us practitioners when we are facing unreasonable opposition who refuse to accept the good story of what makes a better city, but will at least grudgingly won’t have an argument against numbers (unless they are of the utter conspiracy froot loop type).

          So, we need to have less reliance on models, but we can’t get rid of them completely. The primary problem with the models, as I see it, is the ‘fixed demand’ assumption with minimal or no ability to model how people actually make decisions AS A GROUP and OVER TIME. We look at how individual make a decision once. This is more like the 4-step where you decide to take a trip, are generally wedded to a mode and time and you’re just going to take the shortest trip. But, models aren’t sophisticated enough to take into account that you’ll learn from your horrible experience and not make the trip next time, or travel off-peak (most models only look at peak 1-2 hours), change mode, move closer to your work, etc. They also don’t take into account that different people have different limits of tolerance for traffic AND their tolerance changes depending on the trip. Most people may not change their travel behaviour in the face of traffic, but enough will that if you add or remove capacity, demand will adjust. This is how induced demand works, which has been well-proven and yet is almost completely ignored in the field of traffic engineering. When I was studying at uni almost two decades ago this was Traffic Engineering 101, yet in the field of practice we have been sticking our heads in the sand for decades. I suggest also looking up Braess’ Paradox – removing a link in a network (many types of network) often improves overall performance. 
          There are two concepts we need to focus on: 1) Induced demandWe need to be normalising the discussion about induced demand. Everyone should know and understand induced demand. If we are going to improve our models, they need to be based on induced demand. Our political conversations must be based on induced demand. Everyone must be educated – politicians, activists, journalists, planners and engineers. 
          2) Map is not the territory A model is not reality, but a conceptualisation of some aspect of reality. We need to focus on the fact that models are based on only a single set of assumptions about how the world could react to changes, but in reality the world is far too complex. Therefore, models need to be backed up with real world info, as @Heidi points out. We need to continually test our models with reality. Do trials and see how they work. This ties in with point 1 because they will bolster the induced demand argument. We need to make it clear that we have the power to reduce traffic and congestion, not through more capacity, but less. 

      2. Isn’t his comment based on the assumption that congestion will reduce with more lanes, which is pretty much what TDMs that don’t have scope for induced demand will tell him?

    2. Moar roads. brought to you by this Labour-Green government, doubt they can blame Winston for this one.
      It wasn’t too many days ago that it was announced the tracks would be lowered where they pass under the motorway at Drury. I thought this was being done because the motorway work wouldn’t be done on time. Does this new announcement mean it will?

    3. For me, Parker is more disappointing than Twyford. Many of us knew Twyford was a bit of an egg before he was elected, plus we knew he put as much weight on the ‘out’ (sprawl) as well as the ‘up’.
      But Parker? Full of contradictions, that guy. Including talking up the value of public participation then overseeing a NPS-UD that tramples on it…

      1. Tonight he’s spoken of the decimation of Pacific cultures by climate change as if this is isn’t something the world could prevent if politicians got their act together. In today’s SH1 news he showed he chose to listen to the NZTA instead of to his own ministry officials and has chosen to fund increases in transport emissions .

        A Minister for the Environment needs to be less captured by the political economy of car dependence and more focused on climate action and climate justice.

  5. Didn’t even need to get past ‘101’ paragraph to recognise the obvious flaw in the process – Step 3.

    ‘ The third step is to guess how they will get there,’…..

    If car is currently the best option, then car will win every time, and therefore car will get the most attention. No wonder PT struggles to get a chance.

  6. The problem we have is there are multiple interacting parameter in our transport flows. You can’t change one thing without some other unintended outcome cropping up somewhere else. And this is very hard to model. So it comes back to whether we should treat planning and modelling as a science or an art. I would suggest we should treat more as an art than a science. We should not treat it as a religion because then things get very political and projects just get stalled. The CRL is a good example almost 20 years from inception to completion is just not good enough. The art approach would have being to build a light railway from Britomart Station to Mount Eden Station which would have done a similar job in a fraction of the time and cost and not to mention all the congestion we have had to put up with while we have waited and waited for it. And then what if the modelling for the project is wrong and what if things change we could have something like a virus completely upsetting our lives and transport pattern for instance. So the art is to do what is politically and physically possible to give us the best outcome in the shortest time. So say we had taken that approach and now our light rail is reaching capacity what we do. Well we would just build another one after all there are more routes with heavy passenger flows than just Queen Street in the CDB.

    1. Royce, while a tram from Britomart to Mount Eden would certainly fulfill a need, it would in no way “do a similar job to” un-blocking the existing regional metro system with the City Rail Link. Auckland needs the CRL, just as Sydney and Melbourne need their city rail-loops, and Brisbane it’s cross-city rail-tunnel. Many cities with existing rail systems have proved the need for ‘CRL’-type developments. Auckland’s big mistake was abandoning its earlier CRL project in the 1970s. This was not the fault of science, nor art, but of politicians.

      1. We are in rare agreement Dave. Light rail from Mt Eden to Britomart serves a very real purpose, but it is nothing like the purpose of the CRL and is nowhere near adequate to replace the CRL. Realistically we need bothas there simply isn’t the capacity to get people into, out of, around, and across the city centre without both.

        1. 97.5% of what you say I agree with. But I tend not to comment when I agree. I just quietly say to myself, ‘Good on yer, mate’ !

  7. A short-term fix would be to require the models to be published (e.g. during public consultations) along with their results. In academia you can’t publish anything without giving the details, and increasingly you need to provide the actual code and data as well.

    1. This is a really good point. A lack of transparency and understanding of how the modelling works is a huge problem – even within the transport sector.

      1. Transport Assessments for public and private plan changes to the unity plan are published on the council’s website.
        For example, PC51 at Duty South has over 50 pages of transport info here, for people to submit on now…
        Seems like a good amount of info for those interested to get their teeth into to me.

    2. That is a good point. When the ART model was developed by the ARC they did a number of technical papers that were available to anyone who was interested. Unfortunately it was before the internet took off so you had to ask for a paper copy and you didn’t always know what was available. As lawyers like to say sunshine is the best disinfectant.

  8. Transport modeling needs to be updated to consider:

    -Effect to surrounding due to noise, pollution and safety
    -Street design for comfort, calm and relaxed environment for walking and cycling
    -Encouraging opportunities for high amenity high vibrancy Transit oriented development at urban nodes
    -Safety design for children to walk to school

  9. What the Vice article fails to acknowledge is that the four step process is hardly the leading edge in modelling and the limitations of it have been acknowledged for a long time. Beimborn penned a critique back in 1995 that is still widely available on the net.

    A more balanced discussion of the topic would consider whether the more modern activity based demand models are up to the task, and whether more advanced models like that are being used properly and enough.

  10. We noticed this with teh NZTA modelling around the Basin Bridge debacle a few years back. There were two different software packages being used by NZTA and Opus – can’t remember their names – but the results that came out were quite different. Same data went into the two models, but different results came out – which made me realise how much smoke-and-mirrors stuff was actually going on. One of the things I found interesting was that one of the models would only count the cars that actually got through the traffic lights – and so it ignored the half a kilometre or more of cars backed up at that same set of lights. Of limited value, it would seem to me, when it came to “solving” the traffic congestion…

    And then again there is the Embarcadero Expressway example… I’m sure I hardly need to mention that this important piece of urban infrastructure collapsed in the San Fran earthquake – and instead of ensuing chaos the next day, the traffic simply found another way. My guess: probably didn’t comply with the traffic modelling as a result…

    1. And the wonderful absence of the Embarcadero Expressway has so markedly improved that area of San Francisco and increased its rateable value too. We must be very careful about building overhead expressways if we wish to maintain the quality of the areas in which the are built, The same comment applies to to the that awful expressway in Seattle which I think is now demolished since I was last there.

  11. I’m pretty sure the modelling carried out to reduce Quay St to a single lane each way would have shown carnage on parallel routes. Unfortunately (or futunately) the parallel routes were also getting stuffed by CRL works. Be good to find out what has actually happened compared to what was modelled. Quay has been a single lane each way for some time now.

  12. I’ve wondered this when looking at the modelling around cycleway usage and planning.

    To my eye, for example, the current modelling radically underestimates the influx of e-bikes and their effect (on both demographics and distance people are prepared to bike); doesn’t account for other kinds of micromobility (e.g. e-scooters, mobility scooters, etc); struggles to account for short local bike trips which take other kinds of traffic off the streets (because data is not collected on those); proceeds from a potentially flawed assumption that bike travel is 50/50 commute/ recreational (and that those categories would grow in the same way); and doesn’t necessarily comprehend the network effect (when new connections facilitate more trips).

    And then there’s the matter of how to account for the carbon-reduction benefits, public health benefits, and safety benefits of even a small uptick in meeting the massive and widespread demand for safe space to cycle…

  13. This is purely an observation and not meant with a political agenda, so try not to shoot the messenger.
    If you substitute Climate for Transport in terms of modeling, how does that make you feel?
    What sort of projections and timeframes are they using?
    Do they have narrow focus? Which one is more complex?
    How accurate are the past models when reflecting on data of the past 30-50 years. Cheers.

    “… But the problem of all modeling is that to show the effects of a proposed action, you have to assume that everything else in the background will remain constant, or at least will continue changing only along predictable paths.

    … So perhaps modelling isn’t complete junk science. But always be enormously sceptical when people appear to be abdicating responsibility for a decision to modelling. And always question whether the modelling is simply guiding us to repeat the mistakes of the past. (global population, food supply, peak oil modeling etc.)”

  14. Perhaps we should change the ubiquitous “TDM” to mean Travel Decision Modelling. This should require examination and selection of the First Principles of home, employment, education and shopping choices that precede going outside to travel somewhere. Planting houses instead of vegetables does need some modelling before accepting that it is a good idea. Then some academic rigor can be applied to the choices of travel by destination and mode that can be tested by modelling software, using sensitivity checks to consider what happens if we made choices that didn’t represent what people may actually do. These sensitivity tests need to cover disruptive factors such as e-bikes and new PT modes and networks. Enough calibration should be available from around the world that such tests can be meaningful.
    And broadening benefits assessments to consider the many factors currently not quantified (including Cultural effects) is also necessary for the next step of “Is it worth doing?”

    1. I attended a Complete Streets workshop not too long ago, one of the points the presenter made was that when he has designed town centre upgrades, he has never engaged traffic modellers. When the councils have asked him why, he simply says, “Because I don’t care about cars – town centres are for people, not machines.”
      And of course his town centre designs work wonderfully for pedestrians, cyclists and surprise, surprise, retailers.

  15. This article is complete tripe.

    “For transport, models make travel demand seem like a completely external force that must be predicted and then provided for. This approach is central to the thinking that has got us into the congested, unsafe, unsustainable mess we find ourselves in. ”

    The reason we are in a “mess” is:
    1) The failure to properly price transport – no congestion tolls, subsidised partking, no vehicle air pollution levy, underpricing RUCs for trucks etc.
    2) In many cases the total and complete lack of integration of land use and transport planning. Planners zone sprawling low density housing – what other outcome could be expected than car dependency.

    Travel demand is nothing more than the result of the spatial distribution of land use and the costs faced by transport users for various modes.

    Blaming the tool used to estimate the demands is a joke, and an abdication of the responsibility for making the proper but real hard choices and transport pricing and land use distribution.

    1. I agree.
      And it smacks of a refutation to accept analysis that leads to conclusions that don’t suit you. That’s something that is very dangerous.

      Question analysis and investigate it by all means, but don’t completely write it off.

  16. Good article thanks.

    If you have played SimCity or Skylines like me, you soon get to see that using historic data & models only gives you some idea of what will happen when you change something. Playing this game you get to be traffic engineer, urban/town planner, major, politician, accountant, economist & God all at once really. You can actually change where people will live, work, shop & play and not just respond to current demand on one road or transit line.

    So we need to not loose sight of what Jarred Walker says on this one:

    “.. commenting on the same article in a recent post, highlights that modelling can be useful when used properly – but it should never be the decision-maker and it should never be seen as simulating exactly what will happen.”

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