These gravitation effects are often difficult to quantify, and in order to detect any significant trends the phenomenon has to be monitored for longer time periods. Likewise, different type of counteracting effects should be considered, like the cost of wear and tear, and the need for investments in infrastructure. To conclude, the authors were critical towards some of the simplified interpretations of economic impact studies, but still argued that such studies are needed. However, these studies should take account of all effects, both positive and negative effects on regional economies, include comparative analyses (i.e. what would have been the effects of alternative investments), and also explicitly specify the type of financing allocated to the project under consideration.