Cost-Benefit Analysis for Large Infrastructure, What Good Looks Like
Cost-benefit analysis is not an academic exercise. In every serious public infrastructure investment framework, from the National Treasury Budget Facility for Infrastructure in South Africa to the World Bank's public investment appraisal guidance, CBA is the discipline that separates infrastructure investment from infrastructure spending. Investment produces returns to society; spending does not. The CBA is where the distinction is proven or disproven.
This piece sits under the m1 pillar on what makes a large infrastructure project fundable. It sets out the difference between an economic CBA and a financial CBA, the shadow pricing decisions that drive the answers, and what a CBA that will survive National Treasury BFI review looks like.
Financial CBA versus economic CBA
The starting distinction is between the two kinds of CBA, and the mistake is applying one where the other is required.
Financial CBA measures the project from the perspective of the sponsor or investor. It uses market prices for inputs and outputs, applies the sponsor's cost of capital as the discount rate, and produces metrics (NPV, IRR, DSCR) that answer the question: is this project financially attractive to the sponsor at market pricing?
Economic CBA measures the project from the perspective of society at large. It uses shadow prices where market prices do not reflect true social value, applies a social discount rate (often lower than the sponsor's cost of capital), and produces metrics (economic NPV, benefit-cost ratio) that answer the question: is this project socially valuable at true resource costs?
The two are not substitutes. A project can have a strong financial CBA (the sponsor makes money) and a weak economic CBA (the project destroys social value through externalities or displacement effects). The reverse is also possible: a project with modest financial returns can have a strong economic case where the socio-economic benefits are significant.
South Africa's National Treasury Budget Facility for Infrastructure requires the economic CBA for public investment decisions (National Treasury, 2010; National Treasury, 2019). The financial CBA is relevant to the sponsor's own decision and to any private participation in the project, but the public investment case rests on the economic analysis.
Shadow pricing, the heart of economic CBA
Shadow pricing is what distinguishes economic CBA from financial CBA. Where market prices do not reflect true resource costs (because of taxes, subsidies, market failures, or non-market goods), shadow prices adjust for the divergence.
The most common shadow prices in South African infrastructure CBA include:
Labour. Where the project employs workers who would otherwise be unemployed, the true cost of labour to society is lower than the wage paid; the shadow price of labour reflects the opportunity cost, not the market wage. The National Treasury guidelines are prescriptive about the shadow labour wage rate to be used in submissions.
Foreign exchange. Where the exchange rate does not reflect the true scarcity of foreign currency (because of trade restrictions, capital controls, or systematic misalignment), a shadow exchange rate adjusts. In South Africa this adjustment is typically modest for a floating exchange rate; it is more material in economies with managed rates.
Time saved. Transport projects generate benefits by reducing travel time. The shadow price of an hour of time saved varies by user type (commuters, freight, business travellers) and by region. National Treasury's guide provides specific values.
Statistical life. Safety improvements save lives; the shadow price of a statistical life saved is used to quantify this benefit. The value is contested internationally; South African CBA typically uses values in a defined range, often anchored to National Treasury guidance and to World Bank benchmarks (World Bank, 2017).
Carbon. Where the project affects greenhouse gas emissions, a shadow price of carbon is applied. This has become increasingly important as climate considerations enter infrastructure appraisal, and the shadow price of carbon is one of the fastest-moving assumptions in contemporary CBA practice.
The economic CBA's credibility rests on the transparency of the shadow prices used and the sources cited for each. A CBA that presents shadow-priced values without disclosing the specific prices and their sources cannot be inspected by the evaluation panel.
The socio-economic overlay
Economic CBA in South African public infrastructure includes a socio-economic overlay: additional considerations that go beyond the strict quantitative benefit-cost analysis but that inform the public investment decision.
The socio-economic overlay typically covers:
Distributional effects. Who benefits and who bears the cost? A project that generates large aggregate benefits but concentrates them among already-affluent groups scores differently from a project that generates similar aggregate benefits distributed to disadvantaged communities.
Employment intensity. How much sustainable employment does the project create, at what skill levels, and in what regions? Projects in high-unemployment regions and projects that develop skills that increase workforce mobility carry additional weight.
Environmental and social safeguards. Beyond the shadow-priced environmental effects in the quantitative CBA, the project's compliance with environmental impact assessment requirements and its treatment of affected communities are considered qualitatively.
Strategic alignment. Does the project align with the country's strategic priorities as expressed in the National Development Plan and provincial infrastructure intents? Projects aligned with declared strategic priorities receive additional support.
The National Treasury BFI framework integrates the quantitative CBA and the socio-economic overlay in a defined evaluation methodology. Submissions that treat the two as separate exercises miss the point; they should be integrated so that the socio-economic considerations inform the CBA assumptions and vice versa (National Treasury, 2019).
Strategic futures and long-horizon CBA
For infrastructure with long operating lives, the CBA needs to accommodate significant uncertainty about the future context in which the project will operate. Traffic forecasts thirty years out, energy demand forecasts twenty years out, and water demand forecasts fifty years out all sit in a range of plausible futures rather than at a single point estimate.
The strategic futures discipline (the subject of category m5 in this Insights series) provides tools for handling this uncertainty. Scenario-based CBA presents the project's economic case under multiple internally-consistent future scenarios, rather than at a single central case. Where the project's economic case holds across all plausible scenarios, the CBA is robust. Where it holds only under specific optimistic assumptions, the CBA is fragile.
The Western Cape Strategic Infrastructure Intent (2026 to 2050) integrates scenario-based futures work with infrastructure planning in exactly this shape: infrastructure decisions taken in the context of a defined set of plausible futures rather than in the context of a single forecast. This approach is more demanding than single-scenario planning but produces infrastructure investment decisions that are more resilient to the uncertainty that necessarily attends long-horizon planning (Adendorff, 2019).
Cross-border considerations
Cross-border infrastructure projects add complexity to the CBA because the beneficiaries and cost-bearers span multiple jurisdictions. A CBA for a shared water resource or a transboundary transport corridor needs to allocate the benefits and costs to the specific countries involved, and needs to establish a consistent shadow-price framework across the jurisdictions.
Multilateral participation (World Bank, regional development banks) often provides the shadow-price framework by imposing a consistent methodology across cross-border projects. Sponsors and government departments preparing cross-border CBAs should defer to the specific methodology imposed by the participating multilateral, and should ensure the shadow prices used are consistent across the affected jurisdictions.
What good looks like
A well-built economic CBA for large infrastructure shows: the economic and financial versions in parallel with the divergence explained; shadow prices sourced from recognised frameworks (National Treasury, World Bank, or peer-reviewed literature) and documented in the assumption register; a socio-economic overlay integrated with the quantitative analysis rather than bolted on separately; scenario-based sensitivity across plausible futures for long-horizon projects; and the BCR presented alongside the economic NPV, with both metrics interpretable against the evaluation framework's thresholds.
A poorly-built CBA does one of the following: presents financial metrics without an economic version, uses shadow prices without sourcing, treats the socio-economic overlay as decoration, or presents the economic case at a single point without acknowledging future uncertainty. National Treasury BFI evaluators, DFI investment officers, and sophisticated commercial lenders can distinguish the two at a glance.
The CentraSolve Large Infrastructure module supports the CBA discipline through the workflow: the economic and financial views run in parallel, the shadow-price register is a first-class artefact, the socio-economic overlay is integrated, and the scenario-based sensitivity is generated from the underlying model.
References
South African framework sources
National Treasury, Republic of South Africa. (2010). A guide to socio-economic cost-benefit analysis for public investments. Pretoria: National Treasury.
National Treasury, Republic of South Africa. (2019). Budget Facility for Infrastructure: guidelines and templates. Pretoria: National Treasury.
International framework sources
World Bank Group. (2017). Public-private partnerships reference guide (Version 3.0). Washington, DC: World Bank.
Yescombe, E. R., and Farquharson, E. (2018). Public-private partnerships for infrastructure: principles of policy and finance (2nd ed.). Oxford: Butterworth-Heinemann.
Strategic futures sources
Adendorff, C. M. (2019). Infrastructure development scenarios for South Africa towards 2050. Prepared for the Presidential Infrastructure Coordinating Commission, Republic of South Africa.