Fourth, the change in the probability of finding a beneficial (detrimental) impact in studies that report absolute losers alongside winners significantly decreases (increases) by 34.7 (18.3) percentage points.

# **6. Conclusion**

This paper conducted a quantitative-based review and formal meta-regression analysis of studies estimating the wider economic impacts of large-scale transport infrastructure projects. The primary objective was to help policy makers better solve the problem of optimally designing the intervention package for a transport corridor. While anchored in the trunk transport infrastructure, such an intervention package also includes a set of complementary policies and institutional reforms that help amplify the net wider economic benefits of the trunk infrastructure by boosting average impacts and mitigating tradeoffs. With this objective, the paper yields several important insights.

First, policy makers must account for potential trade-offs in different development outcomes and across different (sub) sets of economic actors when designing a transport corridor package. This follows from two key findings of the literature review:

- While for the wider economic outcomes of economic welfare and equity, average estimated impacts tend to be beneficial, for environmental quality and possibly social inclusion, they are often detrimental.

- Even for economic welfare and equity, while the average impacts may be beneficial, considerable heterogeneity in these impacts can exist. In some cases, this heterogeneity may involve subnational locations or segments of the population that lose in absolute terms.

The existence and nature of these trade-offs should drive the choice of complementary interventions that accompany the construction of the trunk transport infrastructure itself—such as compensation policies for the identified losers from trunk infrastructure investments.

Second, the set of complementary policies and institutions that form part of the optimal transport corridor package could depend on the nature of the transport infrastructure intervention that forms the backbone of that package. This follows, for example, from our meta-regression finding that estimated impacts depend on the type of locations that are being connected. In particular, transport projects that target enhancing connectivity between urban centers and international gateways (such as ports, land border crossing points, and airports) yield significantly smaller or less certain impacts than projects that target purely internal enhancements of connectivity (such as enhancements between two domestic cities). This is consistent with the hypothesis that realizing the full benefits of better connectivity to international gateways requires tackling impediments imposed by other factors, such as inefficient port operations or the existence of prohibitively high tariff and nontariff barriers to trade. Although not necessarily considered by the surveyed empirical literature, our canonical model of the policy maker’s problem suggests that the set of complementary policies and institutions that enter into the optimal corridor package will depend on two aspects: preintervention conditions (imperfections) in product, capital, labor, and land markets; and the initial endowments of different locations and economic agents that are affected by the transport intervention.

Third, the placement and design of the transport infrastructure itself matters for the net wider economic benefits a transport corridor package can achieve. Not only does this follow from the finding 21