Alkmaar (93,000 inhabitants) was chosen as an model. If bicycle use were to increase by ten per cent, the number of car kilometres in and around Alkmaar would fall by 3 per cent. Within the built-up area this decrease would be even twice as high (minus 6 per cent). There is a particularly steep fall in the number of car journeys to and from the town centre. This shift from car to bicycle has substantial to huge effects for nine out of the ten problems studied. The reduction in the number of serious traffic casualties is the only issue to remain almost zero. The shift from car to bicycle has in particular enormous effects on problems traditionally part and parcel of traffic departments. Parking pressure in the town centre decreases by over twenty per cent, the number of hours lost by an average driver falls by approximately 15 per cent and there are 11 per cent less car kilometres in residential areas. The effects on local environmental problems are intermediate. Cars and lorries emit 6 per cent less fine dust particles and nitrogen oxides within the built-up area. The contributions to the fight against lack of exercise and the greenhouse effect are modest, but still substantial.