the system can get confused when not being informed about proper usage. For example, when Switzerland switched to the European-wide blue parking disc system, the "blue zones" in Basel were marked with "1 hour" signs that previously read "1½ hours", giving the impression that the time was reduced, but in fact it had not been. Another common error is when an overseas tourist might set the clock to the mark just preceding the current arrival time, which can make a one-hour parking zone as short as thirty minutes, possibly resulting in an unjustified parking ticket. On March 31, 1979 the conference of ministers of transportation in the European Union decided that a European standard should only use designs with a single clock face. The standardized clock disc was introduced as federal law in Germany in November 1981, and similar designs were adopted in other European countries. From 1998 the old parking disc designs began to be abolished. France set the last date to 2007, while in Switzerland (not an EU member) the EU parking disc design was introduced in 2000 and the old designs abolished in 2003. Modern parking signs demanding disc parking include a pictogram of the standardized EU parking disc along with a number showing the maximum parking time in hours. Local regulations France The disk-parking scheme was invented in Paris in the 1950s. The number of vehicles in the capital region had increased considerably from 400000 in 1938 to as much as 920000 at the end of 1955. On September 19, 1955 the police prefect ordered to tow off cars that were parked illegally. In parallel investigations were started how to get rid of the problem which he described in a publication on 16 January 1956. He had already started a parking experiment in November 1955 - parking on the Champs-Élysées avenue was limited to 1 hour. In order to control the maximum parking time small pieces of paper were stuck on the outer ring of the tire with the color of the paper changing every hour. This made for an effective parking time between 1 hour to about 1 hour 55 minutes. Additionally investigations on charging meter options were started in July 1955. The experiments, interrogations and studies lasted well into 1957 when the first "zone bleue" (blue zone) was installed in the East of Paris center (north of the Seine with boundaries at Place de