The city suffers from traffic problems largely due to this radial street pattern, making it difficult for Romans to move easily from the vicinity of one of the radial roads to another without going into the historic centre or using the ring-road. These problems are not helped by the limited size of Rome's metro system when compared to other cities of similar size. In addition, Rome has only 21 taxis for every 10,000 inhabitants, far below other major European cities. Chronic congestion caused by cars during the 1970s and 1980s led to restrictions being placed on vehicle access to the inner city-centre during the hours of daylight. Areas, where these restrictions apply, are known as Limited Traffic Zones (''Zona a Traffico Limitato'' (ZTL) in Italian). More recently, heavy night-time traffic in Trastevere, Testaccio and San Lorenzo has led to the creation of night-time ZTLs in those districts.