Inaccuracy of traffic forecasts and cost estimates on large transport projects

Little research has been carried out on before-and-after studies of traffic and costs in large transport infrastructure projects. The few studies made all show a tendency for forecasts of construction costs to be underestimated, and for traffic forecasts to be overestimated. An examination of construction costs and traffic in seven large Danish bridge and tunnel projects shows that construction costs have been underestimated and traffic has been overestimated in the initial phases of planning. This pattern is also found in studies from other countries of large transport infrastructure projects. When the results of the Danish and the other projects are pooled together, the main lesson to be learnt is that cost overruns of 50-100% are common and overruns above 100% are not uncommon. Traffic forecasts that are incorrect by 20-60% compared with actual development are common in large transport infrastructure projects. The result of this overoptimism in the initial phases of planning is that decisions are based on misleading forecasts that may lead to a misallocation of funds and underperforming projects.