Proposals to avoid environmental damage by re-routing or changing the scope of road or railroad projects are often rejected by developers on the grounds that changes are too expensive. However, through running and sharing in-depth economic analyses that properly calculate financial costs and quantify tangible environmental, economic and social externalities, it is often possible to persuade decision-makers to choose more environmentally-friendly project alternatives. We describe two cases in which the most economically viable option also avoids biodiversity impacts. First, valuation of potential revenue loss from Gorilla tourism in Uganda's Bwindi Impenetrable National Park showed that road alternatives outside the park's boundaries have better overall economic performance than upgrading a route through the Park. Moreover, road alternatives outside the Park would avoid further fragmentation of one of the last remaining habitats of the critically endangered mountain gorilla (Gorilla beringei beringei). Second, we present an economic analysis of proposed road and rail projects connecting Pucallpa (Peru) to Cruzeiro do Sul (Brasil). This analysis shows that neither transport alternative is economically viable. This finding holds even without incorporating the value of negative environmental effects; project investment and maintenance costs simply outweigh reduced transport costs and projections of increased profitability of economic activities in the area. As with the case in Bwindi, the best economic choice (in this case not building the road or railway) also avoids significant risk of deforestation and fragmentation in high biodiversity areas. We conclude that there is significant scope for avoidance of biodiversity impacts through targeted, up-front environmental economic analyses. This work was part of a four-year USAID funded program entitled Biodiversity Understanding in Infrastructure Development (BUILD), with activities in Andes-Amazon, Albertine Rift and Himalayas.