Variable speed of light contribute to the above refractive index, Dicke considered a cosmology where "c" decreased in time, providing an alternative explanation to the cosmological redshift (p. 374). Dicke's theory is compatible with the SI definition of c since the time and length units second and meter can vary in concert, keeping the value of c fixed (p. 366). Spatial variation of the speed of light is implicitly present in general relativity, occurring in the coordinate space description.: "note that the photon speed is ... formula_9." (formula_10 denoting the gravitational potential −"GM"/"r") Based on this observation, variable speed of light models, including Dicke's,
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