I’m probably going to write this up more precisely with Eric and send this to a journal, but for now, I’d like to use our construction costs database to discuss the cost ratio of subways to elevated lines. The table I’m working from can be found here; we’re adding projects and will do a major update probably at the end of the month, but I don’t expect the new data to change the conclusion. Overall, the data is consistent with a subway : el cost ratio in the 2-2.5 range, but it’s not possible to get more precise estimates despite the breadth of the data.
Our database has 11,559 km of total length, but not all of that comes with cost estimates yet; subtracting lines for which we don’t have costs, we get 11,095 km. The total cost of all the lines in our database is, in PPP-adjusted but not inflation-adjusted dollars, $2.302 trillion, for an average of $207 million/km. Nearly all of the items are recent – the majority by length are still under construction, and only 10% opened by 2010. So inflation adjustment is minor, though nontrivial.
Moreover, looking only at 100% underground lines, we get 3955.3 km, for a cost of $945.3 billion, averaging $239 million/km. The other lines are mixed or elevated. The purely elevated lines total 2490.4 km, for a cost of $408.1 billion, or $164 million.
To be slightly fancier but use the same underlying data, the linear estimate of cost per km, treating the tunnel proportion as the independent variable, is 153.1406 + 117.5787*tunnel-proportion; this has a larger spread than just averaging pure subways and pure els, coming from both the inclusion of more data and from not weighting by line length.
However, even the larger spread has a subway : el cost ratio of 1.77, lower than found elsewhere in the literature. Why?
Els are disproportionately build in higher-cost countries
The most important quantitative fact coming out of the analysis of construction costs is that the most important independent variables are country-level dummies. The correlation between the tunnel proportion and cost per km is just 0.163; the correlation between cost per km and a dummy variable that takes the value 1 in the US, Canada, Britain, Australia, New Zealand, and Singapore and 0 elsewhere, is 0.543. If we instead set the dummy variable to take the value 1 in the countries I consider cheap – Spain, Portugal, Italy, Greece, Bulgaria, Switzerland, Sweden, Norway, Denmark, Finland, Turkey, South Korea – then the correlation with plain cost is -0.18, and since linear correlation is better at detecting high outliers than low-but-positive ones, we can take the reciprocal of cost and then the correlation is 0.258.
So it’s useful to figure out where the most els are being built. For example, China has 5,933 km in our database – that is, a slight majority – of which 3,851 are confirmed tunnel and another 1,046 are unconfirmed (Hangzhou in particular is bad about reporting tunnel proportions). Excluding lines with unconfirmed information, we have 9,842.6 km of which 6,436.4 are in tunnel, or 65% – but China is 3,851/4,887, or 79%.
In the lowest-cost countries, els are not common. In Spain, 205.7 km out of 253.8 in our database are underground, or 81%. The Korean lines in our database are 100% underground, and as we add more data, this will hardly change. Overall, the countries I consider cheap have 927 km of rapid transit in the database, which number will rise as we add more Korean data, and of those, 730.1 are underground, a total of 79%. What’s more, one third of the non-underground length in cheap countries consists of a single 63 km item, tagged CR3, consisting of surface improvements for Marmaray (the tunnel is costed separately, as BC1); 63 km is hefty, but as a single item, it is less visible to unweighted correlation estimates like the regression.
So if els are uncommon in China and in cheap countries, where are they common? The answer is high-cost developing countries and Gulf states. India has 1,046.7 km in the database, of which only 235.8 are underground, or 23%; when I continue my series of posts on rapid transit traditions and get to India, I will of course mention the predominance of els. Moreover, these Indian els are spread across many items – there are 29 Indian items, since individual lines in Mumbai and phases in other cities each get their own lines, which matters for unweighted correlation estimates. Similarly, Thailand is 20% underground, Vietnam 50%, Pakistan’s single line 6%, Bangladesh 48%, the Philippines 55%, Malaysia 22%, Indonesia’s single line 38%, Panama 12%, Saudi Arabia 14%, the UAE 22% – and all of these are high-cost. In the developed world, the el-happiest country is Taiwan, only 40% underground in our database, and it’s on the expensive side, its average cost at 40% underground still amounting to $240 million/km, and its three all-underground lines averaging $375 million/km.
It makes sense when you think about it. If construction costs in a country are higher, then it will look for ways to cut costs by building less visually desirable els (typically in developing countries) or slower light rail lines (as in the United States). If we included at-grade light rail lines, then our table would also have a wealth of high-cost American lines; as it is, we’re likely to add some at-grade heavy rail lines like the Silver Line in Washington and, if it actually begins construction, the planned PATH extension to Newark Airport.
So instead of averaging in the entire database, let’s look internally to countries, chosen to be big enough to have a mix of projects with different underground proportions. I’m also going to ignore some cases where I worry about comparability – for example, in France, above-ground lines are represented mostly by a metro extension in Toulouse and by the most outlying parts of Grand Paris Express, and I worry about comparing those with Parisian and inner-suburban tunnels. The worst exclusion has to be that of China: while there is a wealth of data there, China built more els 15-20 years ago than it does now, so comparing subways to els in (say) Shanghai is to some extent a comparison of costs in the 2010s to costs in the late 1990s and early 2000s. In that, China is hardly different from the United States – New York built many els from the 19th century until the mid-1920s, but subsequently built an almost 100% underground system.
In Japan, we go back to the 1990s, so using dollar amounts does have inflation artifacts. Thankfully, the yen has had no inflation, so we can just plug in raw yen numbers and convert at the 2020 rate of 100:1. The 100% underground lines in Japan have averaged $382 million per km, the elevated ones $123 million/km; the ratio is 3.1. The regression estimate, again using ¥100 = $1 throughout, is cost = 149.8978 + 255.9496*tunnel-proportion; the ratio using this method is 2.7.
India has a single 100% underground line in the database, Line 3 in Mumbai, built for $449 million/km. The pure els in India cost $158 million/km, for a subway : el ratio of 2.8. Looking only at els in Mumbai, the average inches up to $167 million/km, a ratio of 2.7. Inflation adjustment would have marginal impact as all of these lines are recent, the earliest priced in 2011 terms. The regression estimate (for all of India, not just Mumbai) is cost = 151.6146 + 222.2716*tunnel-proportion, which yields a ratio of 2.5.
As mentioned above, Taiwan’s three pure subways average $375 million/km. But as a note of caution, they are all regional rail tunnels, and we know from evidence in countries that build 100% underground metros and regional rail tunnels (Finland, Sweden, France, Britain, Germany…) that the latter are more expensive.
With that caveat, the four pure above-ground lines in Taiwan average $170 million/km, a ratio of 2.2. The regression estimate is cost = 183.3252 + 163.0895*tunnel-proportion, a ratio of 1.9. This is a lower ratio than in India and Japan, despite the caveat; the reason could be that the underground lines in the dataset are in Kaohsiung, Taoyuan, and Tainan, whereas the lines in Taipei and New Taipei are elevated, as the database so far does not include the older Taipei MRT lines with their city-center tunnels.
There are no pure subways in Thailand; even the underground MRT’s extension is only 20% underground. However, the under-construction Orange Line is 75% underground, and costs $531 million/km. Overall, the regression estimate is 155.9491 + 350.2821*tunnel-proportion, which includes a number of lines in Bangkok and a cheaper half-underground line in Chiang Mai. This is a ratio of 3.2; excluding the one Chiang Mai line, this rises to 3.9.
Our database is consistent with the observation in the literature that the subway : el cost ratio is about 2-2.5. But a crude averaging of global costs would lead to an underestimate, since higher-cost countries are more likely to be building els. This is partly coincidence – former colonies in the developing world tend to have high costs and also wide throughfares where els are more politically acceptable – and partly the use of els to reduce costs where the country’s ability to afford subways is limited.
This reinforces the need to look at other treatments for reducing costs more carefully. It’s plausible that some policy treatments are not found in low-cost countries because those treatments are undesirable for some reason but do reduce costs. Thus, it is critical to look at both the best industry practices and the variation in practices within the parts of the world one considers best.