Speed of light with their product equalling "c". In 1983 the metre was defined as "the length of the path travelled by light in vacuum during a time interval of of a second", fixing the value of the speed of light at by definition, as described below. Consequently, accurate measurements of the speed of light yield an accurate realization of the metre rather than an accurate value of "c". Outer space is a convenient setting for measuring the speed of light because of its large scale and nearly perfect vacuum. Typically, one measures the time needed for light to traverse some reference distance
Road speed limits in the United Kingdom Road speed limits in the United Kingdom Road speed limits in the United Kingdom are used to define the maximum legal speed (which may be variable) for vehicles using public roads in the UK, and are one of the measures available to attempt to control traffic speeds. The speed limit in each location is indicated on a nearby traffic sign or by the presence of street lighting. Signs show speed limits in miles per hour (mph) or use the Clearway (or national speed limit (NSL)) symbol. The national speed limit is 70 mph (112 km/h) on motorways, 70 mph (112