Where I live in Iowa I found some interesting insight into what impact fees are set in place and what causes the impact fees to be necessary. When a new development is proprosed within the Impact Zone, the Trip Generation Manual published by the Institute of Transportation Engineers (ITE) is used to estimate the number of trips based on the proposed type and size. This total number of trips is multiplied by the per-trip fee calculated by the study to determine teh total Impact Fee for the new development. The fee is paid when a building permit is issued by the County Planning and Zoning Department or the Town Department of Development. The Impact Zone study calculates a $268.17 Road Impact Fee and a $4.25 Culvert/Bridge Impact fee, for a total pre-trip fee of $272.42. Trip estimates for commercial and industrial developments can vary significantly based on the size and proposed use of the site and buildings, so these fees are not determined until a specific development is proposed