Last week, I took a look at some new research from the Netherlands that estimated the benefits of public transport for car travel times based on data from 13 “natural experiments” – public transport strikes. The Dutch researchers found that PT provided significant congestion reduction benefits – around €95 million per annum, equal to 47% of PT fare subsidies.

While the data was specific to Rotterdam, I’d expect to find similar results in most other cities with half-decent public transport networks. The whole thing got me wondering: Is there any similar evidence from New Zealand?

Fortunately for PT users and drivers, but unfortunately for researchers, potential PT strikes have mostly been averted over the last few years. However, Wellington did experience a “natural experiment” of sorts back in June 2013, when a major storm washed out the Hutt Valley railway line:

Source: NZTA
Source: NZTA

The Hutt Valley rail line was out for six days, including four working days. During that period, things got pretty ugly on the roads, as the motorway into downtown Wellington didn’t have enough capacity to accommodate people who ordinarily commuted in by train.

The Ministry of Transport (among others) very cleverly observed that this was a great opportunity to learn something about the impact of PT networks on road congestion. During the rail outage, they surveyed around 1,000 Wellington commuters about their travel experiences. According to their report, they found that:

  • The closure of the Hutt Valley rail line put significant pressure on the road network. Delays for commuters were most severe on the Monday following the storm. Traffic on State Highway 2 was severely congested, with morning peak hour conditions lasting two hours longer than usual
    • 80 percent of Wellington commuters from the Hutt Valley and Wairarapa experienced a longer than usual trip
    • 32 percent of them experienced delays of over an hour
  • the severity of commuter delays lessened over the week, with the number of commuters from the Hutt Valley and Wairarapa experiencing delays of over an hour halving by Wednesday 26 June

Essentially, what happened was that a bunch of people who ordinarily caught the train from the Hutt Valley couldn’t do that due to the storm damage. A quick eyeballing of MoT’s graph of daily rail patronage suggests that around 4,000 people had to make other travel arrangements:

Wellington storm daily train patronage

Almost half of the rail commuters from the Hutt Valley opted to drive instead, while the remainder chose to take replacement buses or to stay at home instead. This had a serious impact on motorway traffic, as shown on this graph of hourly southbound traffic volumes. On a normal day (the green or blue lines), traffic volumes peak at around 7-8am, and fall off sharply after that.

By contrast, on Monday 24 June, when the rail line was out, people were still travelling in (slowly) until almost 11am. That’s some serious congestion:

Wellington storm hourly vehicle flows

Based on survey data, MoT estimated that the storm damage increased average travel times during the morning peak by 0.329 hours (20 minutes) on Friday 21 June, 0.309 hours (18.5 minutes) on Monday 24 June, and 0.230 hours (14 minutes) on Wednesday 26 June. It then used those estimates of average delay for people travelling at peak time to estimate the added cost of congestion that arose as a result of the Hutt Valley rail line outage:

Wellington storm cost of increased travel time

In short, a four-day breakdown in part of Wellington’s public transport network cost morning peak travellers around $2.66 million in lost time. If we assume that there was a similar level of delay during the afternoon peak, when people are commuting out of downtown Wellington, the total cost would be roughly double that – $5.32 million.

This can give us a rough estimate of the value of public transport for congestion relief in Wellington. Extrapolated out over a full year (i.e. 250 working days), these results suggest that the Hutt Valley rail line saves drivers the equivalent of around $330 million in travel time (i.e. $5.32m / 4 days * 250 working days).

That is a very large number. According to an Auckland Transport report comparing Auckland and Wellington rail performance, Wellington’s overall rail network only cost $81.2 million to operate in 2013. 56% of operating costs were covered by fares, meaning that the total public subsidy for the network is around $36 million per annum.

On the back of these figures, it looks like Wellington’s drivers are getting a fantastic return from using some fuel taxes to pay for PT rather than more roads. The travel time savings associated with the Hutt Valley line alone are nine times as large as the operating subsidy for the entire Wellington rail network.

There are two caveats worth applying to these figures, one practical and one methodological.

First, it’s likely that the value of rail for congestion relief is unusually high in Wellington due to the shape of the city. Here’s a map of Wellington’s population density and infrastructure in 2001 and 2013 (from my analysis of urban population density). Dormitory suburbs extend linearly up the Hutt Valley and towards Porirua and the Kapiti Coast. Everyone travelling from those places to downtown Wellington are funnelled through a single transport corridor running along the shoreline of the harbour:

Wellington density 01-13 v2

In Wellington, losing the rail line means pushing everyone onto a single road. (Unlike Rotterdam, cycling isn’t especially viable due to the lack of safe infrastructure on this route.) In other cities, there tend to be a greater range of alternative routes, which spreads around the traffic impacts.

Second, these results aren’t as robust as the Rotterdam study, due to their use of survey data rather than quantitative measures of traffic flow and speed. They’re not likely to be totally wrong, but it’s likely that people over- or under-estimated commute times, or that the survey wasn’t representative of all travellers (which could invalidate MoT’s extrapolation to all morning peak travellers).

However, the increasing availability of real-time data on traffic speeds from GPS devices means that the next time this happens, it will be possible to measure the impacts in much greater detail and with greater precision. The Rotterdam study offers some good methodological insight into how best to do that – it looks at transport outcomes at specific locations over a long period of time, and controls for seasonal and weekday effects that may influence transport outcomes.

Lastly, it would be really interesting to see some similar analysis done for Auckland. I’m sure that there have been a number of full or partial rail network outages, either due to bad weather or scheduled track upgrades. Perhaps it would be worth taking a look at congestion on those days.

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  1. Great research, but I wonder if a scheduled shutdown of Auckland rail would be a valid comparison as people have time to think about what other options they have and may plan for this. Also because it is short term, people may be willing to do something for a few days that they wouldn’t do long term – e.g. work from home for a number a days on end, cycle on unsafe roads, take a very early or very late bus or car journey to avoid the congestion…

    1. There are many differences between AKL and WTGN, particularly that rail still carries a considerably smaller proportion of total travel here, although it is growing rapidly and a full outage for days would certainly have a non-trivial impact on all other modes. The closest similar event would be if all the buses on the Busway stopped, as the bridge is the most observable route with few alternatives like the Hutt rail/m’way, ferries notwithstanding.

      However there are now some 50k per day using the rail network on a weekday and what? Around 200k on the buses, so whenever even a small proportion of these people decide to drive- like when it rains- driving infarcts all over the place.

      So while it is harder to measure when there aren’t clear parallel routes without alternatives there’s no doubt that Transit users are indeed the driver’s best friend. No Transit; no functional driving, especially at peak.

    2. Good question – only one way to find out!

      I would note, however, that the Rotterdam study dealt with a similar issue – PT strikes are generally scheduled a bit in advance, giving people time to make plans. And both the Rotterdam and Wellington studies found that a significant portion of people either (a) didn’t travel at all on the days of outage or (b) found non-car workarounds.

      My suspicion is that these types of studies understate the decongestion benefits of PT due to the short-term nature of PT outages studied. If PT was permanently unavailable, people wouldn’t have the option of short-term workarounds. (However, on the flip-side there would probably be some land-use responses.)

  2. Great argument. Terrific work. And this barely covers the point that a proportion of people denied train travel still chose public transport – the buses. If those extra bus passengers were in cars the congestion would have been worse, parking in town would have been impossible for some, meaning even more people choosing to stay home and more lost productivity. It also makes the point that separate bus lanes would have given a significant advantage to public transport.

  3. the days bay ferry does rather well when there is an outage on the main route too. a bit of a last resort but it gets you there in the end.

  4. The problem with using the Strike or Natural Disaster disruption effect to justify the decongestion value of PT is that it does not compare apples with apples. The analysis uses the example of the 4 day outage on the Hutt Valley line to say that the rail line has massive benefit in reducing traffic congestion. But although the analysis counts the benefits it does not properly count the costs because it are only compares the transport network having the rail line with the network not having the rail line. It essentially assumes that, should the passenger rail service be halted that other traffic projects would not have been implemented.

    The 2004 Wellington Passenger Rail Business Case that justifiied new trains and renewal of the line outlined that the 25 year public subsidy cost of keeping the Wellington Rail system at $2.4 Billion (2004 prices). About half of this was to support passenger rail services towards the Hutt Valley. This money (and much more) has been invested into the rail network but it could have been invested in alternative transport options.

    Options seriously considered under the 2002 “Hutt Corridor Study Stage 1” including SH2 having a “Hutt Expressway High Occupancy Toll (HOT) Lane” or a “Hutt Expressway Tidal Flow Lane ( Petone – Ngauranga)” either of which could be done for far less than $1Billion (with an extra lane each way from Ngaurunga to the CBD on the plans being implemented) providing traffic decongestion benefits. The Passenger Rail Business Case itself also considered a Bus Rapid Transit alternative.

    Another roading alternative is the Petone to Grenada Link Road ( is also planned and, had it already been in place, the disruption to Hutt Valley commuters from the loss of the railway line would have been considerably less.

    Of lesser importance is the analysis also seriously underestimates the Hutt Valley passenger rail costs. By only considering rail operating costs, it ignores the major rail capital costs both past and future. The Matangi trains that are being operated were not free and neither are their replacements needed in about 23 years time (train depreciation costs are not being funded by the GWRC so they will be back asking for new trains again in a couple of decades). Both these (and many other) major rail capital costs for Wellington passenger rail are %100 public subsidies so I would also suggest the analysis at least double its $36/year estimate of the subsidy cost for the Hutt Valley Line.

    I am, as a PT advocate, not saying that PT doesn’t have considerable decongestion benefits but, IMO, this approach to measuring this benefit is seriously flawed. Although still likely to be positive, the analysis both underestimates rail costs and over-estimates the decongestion benefits and so may have limited value.

    1. I don’t think these objections hold water.

      First, you state that this approach “does not compare apples with apples”. That’s not true. In the Rotterdam study, they controlled for factors such as seasonality, weather, etc, using a decade-long dataset. In the Wellington study, commuters were asked whether they *would have* chosen to travel by a different mode on the particular days in question if it weren’t for the rail outage.

      Second, you argue that we should crank up our estimate of the costs of the Wellington rail system to account for the opportunity cost of foregoing other, alternative projects. (Or simply cutting taxes instead of spending the money.) I’m not aware of any transport evaluation framework that requires this (although it is necessary to make a correction for the deadweight cost of taxation). It’s better to rank projects according to their benefit-cost ratios and then select the projects with the highest BCRs – that way you can be generally confident that you’re not forgoing any better projects.

      Another way of saying that is that the reason those road projects weren’t built at the time was that they simply weren’t as good as the alternative projects on offer.

      Third, while it’s reasonable to include capital costs as well as operating costs, I don’t think that they’d make any difference to the overall conclusion. The monetised decongestion benefits from the Wellington study are *really* large. (Another, slightly more subtle point is that capex costs are largely sunk costs, while we have a choice about whether or not to spend money on opex every year. So if we’re looking to make decisions “on the margin” it’s appropriate to look at opex alone.)

      1. I agree that “It’s better to rank projects according to their benefit-cost ratios and then select the projects with the highest BCRs – that way you can be generally confident that you’re not forgoing any better projects” but this analysis doesn’t do this. It does not compare the rail investment with the next best transport investment options, it compares it to doing nothing which is clearly not realistic.

        You can well claim you “are not aware of any transport evaluation framework that requires this” but this doesn’t make the evaluation any more realistic. If your analysis is claiming to apply to the real world it should recognise that Hutt Valley residents (and the politicians representing them) would not stand for the level of congestion you claim would occur without the rail line … they would do (and are doing) something else to keep the congestion down and retain reasonable (might one say realistic) travel times. Ignore this factor if you wish but it is a valid criticism.

        As for ignoring the major capital costs associated with the Wellington Rail projects well that means you are simply showing the rail line is a key transport service, not that it has any long term decongestion benefit (when SH2 is blocked the trains are also crowded to overflowing … not unexpected but also not justification for the highway either).

        1. I’m not sure what your issue is. MoT’s analysis asks a pretty straightforward and useful question: “Is Wellington’s rail system benefitting drivers?” And the answer seems to be: yes, in spades.

          As far as your point that we could have done something different: do you have any examples of major Wellington road projects with a BCR of nine that aren’t being built? My impression was that they were all struggling to make it over 1.

        2. I agree with Patrick Reynolds’ interpretation of the analysis which is:

          “A PT service was disrupted, what was the outcome to the remaining networks? That’s what was asked and answered.”

          My issue is when the analysis extends this rare (although not rare enough) and short-term cost the rail service failure to incorrectly claim it provides a long term benefit:

          “The travel time savings associated with the Hutt Valley line alone are nine times as large as the operating subsidy for the entire Wellington rail network.” This is wrong in three aspects:

          1) To claim a long term benefit obviously means the analysis should also include the long-term costs which would mean counting both rail capital and operating costs (this alone will halve the claimed nine times BCR).

          2) As ChrisM correctly observed:

          “In the long-term people will change their behaviour (even after just 4 days it looks like the impacts have started lessening), shift into different areas, the roading network would be developed differently… etc”.

          3) As I pointed out, the analysis does not compare the rail investment with alternative transport investment options (such as the Petone to Grenada Link Road) would have been funded had the $Billion or so not been spent on the Hutt rail line, it compares investing in rail with doing nothing which is clearly not realistic over the long term.

          As for Wellington roading project BCRs, in reference to the Petone to Grenada: “When the wider economic benefits of the project are factored in the BCR is over 4.0 for the P2G project as a whole” which makes “The P2G Link Road has the best benefit cost assessment of all New Zealand major roading projects.” (Ref:

          Yes, I am sure the Hutt Rail service is benefiting road commuters but the long term benefit is nothing as great as claimed by this analysis.

        3. Yeah, nah.

          You insist that we double our estimate of the costs of providing rail services due to the capex costs. Fair enough. But you’re ignoring the fact that the storm only knocked out one of Wellington’s regional rail lines. If the Kapiti Line was down as well the traffic impacts could have been proportionately worse.

          The evidence you’ve provided on P2G is not, to my mind, relevant. It is simply too lacking in detail to evaluate whether its claims are correct. In particular:
          * It doesn’t go into any detail about the traffic growth assumptions it’s used, or explained how it’s “ground-truthed” them against what’s actually happening in the region. Wellington traffic volumes have dropped 4% since 2010 and the population growth outlook is pretty anemic.
          * It states that “Option 3 clearly the best option” without mentioning the fact that the incremental BCR (relative to the do-minimum) is only 1.4 – i.e. less than required by NZTA’s incremental BCR policy.
          * It states – without further analysis – that wider economic benefits almost double the BCR for the project. This is implausible. In my experience WEBs typically add more like 10-30% to conventional transport benefits.

          Given your skepticism about the Wellington rail analysis, I’m surprised that you accept the P2G numbers so uncritically.

        4. We have both made our points and I think our respective positions are clear enough. Thanks for the discussion.

        5. Have to agree with Wellington Commuter. Elasticity increases with increasing time horizons. And with such a shock to the system it was bound to take a few days for people to even understand the new situation so they could start adjusting their behaviour. The analysis was worth doing, but the extrapolation to annual de congestion benefits is not convincing.

          What this does show is that rails lack of resilienc compared to road based systems can cause issues in these sorts of events.

        6. Another way to think about this: if we believe the analysis, then each rail commuter is providing in the order of $300 of de congestion benefits per trip. If roads were priced, do we think that the price required to deliver ordinary congestion levels, absent the rail line, is $300? I would suggest at a price of $300 there would tumbleweeds. Conclusion rail may help, but it is a massive second best to road pricing.

        7. Same to you, WC – thanks for the debate!

          Matthew – I agree that there is a long-run/short-run issue at work here. However, I’d observe that transport models used to evaluate new road and PT projects typically reflect short-run costs only (e.g. excluding location choices and vehicle ownership decisions). As a result you could make the exact same critique about the BCRs for new road projects – i.e. “congestion would never get that bad even if we didn’t build the road, because people would choose different modes/live somewhere else”. Regardless of whether we stick with short-run costs or try to model the long-run equilibrium, I think we need consistency!

          Also, I think you’ve got an order of magnitude error in your implied congestion price calculations. The added cost of congestion per AM peak period from the MoT report was around $0.66m. According to appendix A6 of the report, there are an estimated 213,000 car and PT trips in the morning peak. This implies that a congestion price of $3.10 or so per peak traveller (i.e. $0.66m/213,000) would compensate for the outage of the rail line. That seems much more plausible to me.

      2. But surely Wellington Commuter’s point that the impact on congestion due to a 4 day outage is different to the impact on congestion of a long-term absence of public transport is reasonable.
        In the long-term people will change their behaviour (even after just 4 days it looks like the impacts have started lessening), shift into different areas, the roading network would be developed differently… etc.

        1. No WC is trying argue something completely off topic about the particular mode in the study, he is trying to find some evidence in this comparison to push a claim that rail is bad and road is best. It doesn’t wash; there is nothing about this study that cares what the mode is. A PT service was disrupted, what was the outcome to the remaining networks? That’s what was asked and answered. Whether some wheels are steel or rubber is like asking what colour uniform the driver was wearing as if that had any bearing on the outcome. Meh.

        2. I’m not sure if WC is trying to do that or not. I guess I’m just not sure if I agree with Peter’s response above:

          “My suspicion is that these types of studies understate the decongestion benefits of PT due to the short-term nature of PT outages studied. If PT was permanently unavailable, people wouldn’t have the option of short-term workarounds. (However, on the flip-side there would probably be some land-use responses.)”

          I would’ve thought the long-term land-use responses (and transport network responses WC refers to) would be big enough to justify consideration.

        3. There’s still be a loss in welfare from choosing alternative travel patterns or locations, though. This is intuitively sensible – if there were other options that were equally good or better than taking the train to downtown, people would already be choosing to do that instead.

          There are some short-run vs long-run supply and demand curve issues that I’m trying to sort through, as well as questions around externalities (which may vary depending on alternative location choices). But absent a clear analysis of those issues, I don’t think we can just wave our hands and say “it’s all good, people would just live/work somewhere else so the rail line has no benefits”.

    2. Wellington Commuter — re: the Petone to Grenada link road. I’m not convinced that it would have helped in this instance. Some Hutt drivers could have come down the Gorge instead of along the waterfront, but they would have had to fight congestion as far as Petone before they could cut across to SH1, and then both streams would still have jammed up at the SH1/SH2 merge trying to get into the CBD. There is an existing congestion bottle neck on SH1 at Johnsonville, *south* of the proposed P2G join at Tawa. The traffic modelling for P2G shows worsening congestion through Johnsonville after P2G is built on a *normal* day (congestion the project does not intend to address) — the addition of Hutt traffic to that existing bottleneck would have backed SH1 up well through Porirua. P2G would theoretically have helped Petone commuters trying to get north to Tawa or Porirua, but I don’t see how it would have helped anyone headed to the CBD. More streams heading through the same chokepoints doesn’t fix anything.

      As far as the P2G BCR goes, the peer review of previous P2G plans indicates that the current project team is underestimating their earthworks costs by at least a factor of two, and scuttlebutt out of the Transmission Gully project is suggesting they’ve underestimated the rock-bolting alone by a factor of four. So I’d take that BCR with a big dose of salt.

      1. Two points:
        1) The Petone to Grenada Link Road project includes a major upgrade to the Petone Interchange and so congestion at this point would be much better than what happens now.
        2 ) The main purpose of the Petone to Greanda Link Road is to provide a better route for the 20-25% of commuters currently going through Ngauranga but travelling between the Hutt Valley and Tawa/Porirua. While the link road won’t make much difference in the morning for the reasons you point out, the evening commute would be much better because the current congestion at Ngauranga caused by Porirua/Tawa commuters merging with commuters from Wellington City both going to the Hutt Valley will not be there.

        So, although the link road is certainly not a good replacement for the HVL rail service, when it is in place, it will provide some relief from a serious rail service failure … IMO.

        1. Are you saying that 20-25% of the commuters through Ngauranga are “turning the corner”, or that 20-25% of the commuters traveling between Tawa/Porirua and the Hutt do it via Ngauranga? Either way, do you have a citation for that number? We were told that NZTA does not have actual traffic data for where the vehicles traveling through Ngauranga are actually going, so they are having to extrapolate based on census data from people who said they lived in one region and worked in another (the census doesn’t ask what route you take, or whether you travel at peak — I don’t remember it asking about PT usage, either, but I’m a bit fuzzy on that).

          For commuters driving to/from the CBD, they *are* in the process of tacking on extra lanes south of the SH1/SH2 merge, so that’s something. Hutt folks should be aware that the models expect the northbound trip between Ngauranga and Dowse to get worse after P2G goes in. []

        2. I can’t find the source that said that 20% of traffic going through Ngauranga is going to or from Hutt Valley but the Ngauranga Triangle Study ( gives more specific traffic volumes for SH1 & SH2 @ Nauranga:
          ” This part of SH1 carries between 45,000 vpd at Tawa and 70,000 vpd at Ngauranga. Traffic includes significant volumes of heavy vehicles making up 3 to 5 % of the daily vehicle numbers on SH1 in the Ngauranga Gorge. … The segment of SH2 within the study area operates as a four-lane divided expressway and carries 34,000 vpd at Dowse and 67,000 vpd at Ngauranga. Like SH1, this traffic includes significant volumes of heavy vehicles on SH2 north of Ngauranga, making up approximately 10 % of the daily traffic flow.”

          And the modeled traffic relief from theP2G Link Road:
          “The construction of the Petone-Grenada Link is seen as a key way of reducing congestion on SH1 between Johnsonville and the Ngauranga merge and SH2. In the forecast year of 2016, traffic volumes on SH1 at Johnsonville are expected to reduce by 12,000 vpd and on SH2 south of the Petone ramps, by 12,000 vpd if the link road is built. The Petone-Grenada Link Road is expected to carry approximately 25,000 vpd. The Petone-Grenada Link Road also provides some relief to SH58. In the forecast year of 2016, the traffic volumes on SH58 reduce by around 3,000 vpd with the link road in place to 12,000 vpd.”

          And the benefits from P2G including improved reliability:
          “This leads to an am peak SBD travel time savings of 2 minutes per vehicle in 2016 on SH1 between Tawa and Ngauranga and 3 minutes per vehicle in 2026. In the pm peak the NBD travel time savings are minor in 2016 and 2026 on SH1 between Tawa and Ngauranga.On SH2 between Dowse and Ngauranga the time savings generated are offset by the additional delay between Petone and Dowse. This is due to increased traffic volumes on this segment of road.
          In addition to these travel time savings it is expected that the Petone-Grenada Link Road will improve the travel time reliability on both SH1 an d SH2. Additionally, in the event of an incident on either SH1 or SH2 in the study area this road provides an alternative route that will significantly improve the network resilience.”

          As for your comment “Hutt folks should be aware that the models expect the northbound trip between Ngauranga and Dowse to get worse after P2G goes in”, well I reviewed the report and it says:
          “that travel time in the northbound direction overall between the Ngauranga and Dowse Interchanges are likely to reduce by around three minutes.
          However, travel times on SH2 between the Petone Overhead Bridge and Dowse Interchange … are predicted to increase by around one minute in the northbound PM Peak for this discrete section.”

          So the report referenced seems to confirm that, with the P2G Link Road the overall northbound trip will get a bit better not worse ! Or have I missed something ?

  5. So by hypothetical extension, what are the congestion costs of NOT having extended rail along the CBD-Airport corridor decades ago? Or put another way, if this facility had been built back whenever and Wellington had adjusted to its contribution over the years, and then it was taken out of service for a period, what would the costs be?
    And would these costs be a reliable indicator of what we are forgoing every day, as a result of not having the facility?

    And to borrow Wellington Commuter’s argument, how much roading capital (both sunk and future) could we have justifiably saved through adopting this alternative approach?

    All a bit abstract given that to date, we have resolutely chosen to shun this approach, but actually very relevant now that the Basin Flyover outcome has thrown a big spanner in the works of BAU-moar-roadbuilding, and opened the possibility that the basic approach be re-debated.
    Bring it on!

    1. wsomc; interesting thought. What is the investment in parking by the Airports? What business turnover does the Parking generate (include the private parking in Auckland). What Capital investment is there for parking?
      Compare that to the cost of rail link?
      What on going costs do those working at the airport face for parking compared to 24/7 PT? I see PT being extremely beneficial to those working at the airport especially those on minimum wages.

  6. It’s an interesting way of thinking about relieving congestion, but I thought that, due to induced demand (both on roads and on PT), extra infrastructure will not result in less congestion, but in more people travelling during the peak (and as a consequence if you build the infrastructure, you have almost total control over how people will travel. Build only roads, and they will all drive).

    So maybe we could also think of it like this:

    → What would it cost to build a road with the same capacity as those rail lines—and make sure everybody can reach their destination in the city and park his car during the workday.

    Or applied to expanding the harbour crossing:

    more buses
    → What was the cost of building the Northern Busway and operating the associated buses?
    → How many extra people does it allow to commute between the Shore and the CBD?
    vs. more roads
    → What would an extra road bridge (or tunnel) cost?
    → How many extra people would it allow to commute between the Shore and the CBD?

    I’m suspecting the road alternative will come out very badly.

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