This thesis, titled "Theoretical Essays in Transportation Infrastructure," presents three articles on transportation infrastructure and cost-benefit analysis. The first article addresses the distortion of market prices in infrastructure project evaluations, highlighting the risks of incorrect decisions and the waste of public resources. Additionally, it revisits the international literature and identifies gaps in national research, especially in Brazil. The second article investigates the effects of exogenous cost shocks on the competition between Full Service Carriers (FSC) and Low Fare Carriers (LFC) in the Brazilian airline sector, using an oligopoly model and specific demand and cost data. The results show that LFCs suffer greater losses in markup and demand but increase prices proportionally more, creating a predictable environment for consumers. The third article analyzes the impacts of wage premium shocks on the competitive conditions of airlines, using a duopoly model to examine the interaction between a large network carrier and a small low-fare carrier. It concludes that smaller airlines are more affected by labor cost shocks, leading to higher market concentration and highlighting the challenges smaller carriers face in maintaining competitiveness.