The inaccuracy of traffic forecasts has long stood as a central research theme in the field of infrastructure and transportation studies. The literature presents several motives for this phenomenon, ranging from a political bias, insufficient technical preparation, changing urban patterns, and economic dynamics. Uncertainty due to the inaccuracy of forecasts can have a profound impact on the infrastructure development process, right through from the preliminary studies up until the operation and re-negotiation of contracts (in cases when projects are developed using a concessions model). This paper provides an extensive systematic review of forecast inaccuracy in roads and railways projects (analyzing trends, causes, and results). The research found that: (1) forecasts in rail projects are generally more optimistic than in road projects; (2) over the last couple of decades the accuracy of forecasts has not improved significantly, and; (3) there has been a generalized ramp-up effect in forecasts.