If the state spends money on a bad infrastructure project, or too much money beyond what was necessary for the project, then this is waste of money, and should be avoided. But the opposite situation can occur too: some worthwhile projects are not pursued, and that too is a waste, because society forgoes the benefits coming from such projects. This situation should be avoided equally. Moreover, there is no priority between those two types of error. Planning should treat them symmetrically and aim on balance to avoid both equally.
The reason is that just as infrastructure projects are generally not critical, the money that is spent on them is not critical. The US is spending around $1.5 trillion over the lifetime of the program on the F-35 plane, and the money is buried deep in a defense budget that by the standards I grew up with isn’t even that large – and that program consists of documented waste and suffers from poor planning, including serious cost overruns and delays. None of this is an existential threat; the problems the F-35 is intended to solve are not existential but neither are its costs, and likewise, neither infrastructure problems such as delays, capacity limitations, and congestion nor the costs of the projects that intend to fix them are existential.
And if none of this is existential, then the decision of whether to build is about comparing two finite, bounded quantities: costs and benefits. This is why one does a benefit-cost analysis and respect its conclusions, without spiking. But this is also why the state should not systematically aim to err in one direction. If a project with a BCR of less than 1 is built then there is waste, but if a project with a high BCR is not built then there is waste as well.
Note that this principle of not biasing one’s error in one direction (typically the bias is toward inaction) is separate from the question of what the best estimates for costs and benefits are. There is a real tendency to underestimate costs, which is why the minimum BCR that should be funded is not 1 but slightly more, the typical range in Europe being 1.2-1.4. But subject to that limit, decisions should still be symmetric, i.e. if the limit is 1.4, then building 0.7 is symmetrically bad with failing to build 2.8. Alternatively, some projects, like high-speed rail, have upfront costs and long-term benefits, and so it’s better to think of them in terms of financial and social returns on investment, as is done in France (source, pp. 11-12), rather than a BCR in which the discount rate is hidden in a box. But ROI analysis should still be symmetric around one’s chosen limit.
This becomes relevant especially for projects that can expect benefits to rise over time due to economic growth. It is tempting to have a bias toward inaction and only build something once its benefits are unimpeachable, a large multiple of the cost. But this means that in the interim, society has forgone the smaller-but-still-real benefits. Worse, when the BCR grows too large, surplus extraction might pull it back down through an increase in costs, and thus building later can be very risky.
In essence, what this means is that if there’s infrastructure out there with a very high BCR or ROI – and if you ask me, preliminarily, Northeast Corridor high-speed rail done right has a purely financial ROI of maybe 13% – then something is deeply wrong. There shouldn’t be 13% returns out of anything. If there is one, the first question to ask is “why was this not built 50 years ago?”.
In the opposite direction, what looks like building infrastructure prematurely is in fact the prudent decision. South Korea and Taiwan both opened high-speed rail in the 2000s, both underperforming initial expectations. But both have seen steady growth in ridership; at this point, Taiwan HSR returns 4% without social benefits, which is decently healthy, and KTX has somewhat higher ridership than THSR on only slightly higher total construction costs. In the mid-2000s the projects looked like white elephants, that is they were doing just better than minimum. But the 15 years of benefits since then have been considerable. The 20% of society least interested in paying for things should not have veto power; economics exists on the margin and politics on the median.