A new decentralised settling system based on the principle of lamella separation was developed for the treatment of road runoff. Two different laboratory test methods, the DIBt (Deutsches Institut für Bautechnik) procedure and our own approach, were applied in order to evaluate the efficiency of the system based on the separation of fine mineral particles and a mixture of mineral and organic particles, respectively. Overall efficiencies (88% after DIBt and 61% according to our own method) were comparable to results obtained for commercial systems. The lamella system was then applied in the field for 1 year to treat runoff from a road area of 420 m2. The amount of solids separated that was calculated from a mass balance (10.1 kg) was consistent with the amount of sediments measured (8.6 kg). However, the average separation efficiency was only 30% in the field study. This is related to the size and composition of the particles in runoff, which are not represented well by the material used for the test procedures. It is concluded that the test methods should be improved, and that more field studies are needed in order to obtain a better understanding of the settling behaviour of particles in road runoff.