In the 1970s the Dutch city of Delft began restricting private car traffic from crossing the city center.[35] Similarly, Groningen is divided into four zones that cannot be crossed by private motor-traffic, (private cars must use the ring road instead).[36] Cyclists and other traffic can pass between the zones and cycling accounts for 50%+ of trips in Groningen (which reputedly has the third-highest proportion of cycle traffic of any city). The Swedish city of Gothenburg uses a similar system of traffic cells.[37]