An increased amount of ships in the narrow water lanes results in an increased possibility for collisions. This study is illustrating the risk-modelling processes in the scenario of increasing ship activity in Lake Mälaren, central Sweden. About 1.5 million people are daily dependent of fresh water from Lake Mälaren. Meanwhile a lot of hazardous goods and cargos are transported on the lake with all the potential risks that this can bring up. Therefore risk-analysis can be important tools in future policy buildings if it is related to environmental legislations. Cost-benefit and cost-effectiveness is also important tools in allocating procedures when different results are weighted against each other.