Abstract
The Netherlands have a long tradition of using a quantitative risk approach in urban land use planning. By law, local authorities have to establish land use plans. These plans hold the rules, conditions and criteria for land use in order to result in proper spatial planning. In the densely populated and industrialized country of the Netherlands this is not an unnecessary luxury. In the process of establishing land use plans or making formal adjustments to these plans, local authorities have to consider the societal risk, associated with these plans explicitly. For this purpose calculation of the so called F,n-curve is demanded by law. The F,n curve is the quantitative expression of the societal risk. There is a widespread misconception about the use of the F,n- curve by Dutch local authorities. It is not a statutory quantitative risk acceptance criterion. Instead the F,n- curve is one source of information among others to weigh the benefits of a new development against the risks that come with it. The existing curve is compared with the new F,n-curve that incorporates the new development .
Over the last 10 years, experience has shown that government officials consider the F,n-curve as too abstract a piece of information. Its ability to express safety matters relevant for the decision at hand is considered to be too limited. As a result the F,n-curve does not support transparent decision making and weighing the pros and cons of the spatial developments. The authors propose the use of narrative information, derived from the F,n- curve for decision making. This can be done by decomposing the F,n-curve and selecting some representative accident scenario's.
In this way concentric zones may be drawn around the risk source (in case of transport routes parallel zones). These zones correspond to a specific physical effect that may occur after the release of the substance. Because the physical effect results in specific damage consequences it also corresponds to the impact on societal risk resulting from a new development. These possible impacts may be ranked according to their relative contribution to the F,n curve. So in addition to the F,n-curve local authorities may use simple geographic map information linked to concrete damage phenomena. The zones, if selected properly, tell the stakeholders what damage consequences may result from their location choices. The zoning tells them where the impact of a new development on societal risk changes from high to medium, to low and finally can be disregarded and why this is the case. Narrative information derived from the F,n-curve can assist in making decisions more transparent. The crux is that a zone-limit marks the transition in the degree of protection a building offers to the exposed person. The current policy development in the Netherlands is inclined towards a more effects-based approach.
