In the 1980s and 1990s, there was increasing pressure globally from customer airlines for at least some improvements in low visibility operations; both for flight regularity and from safety considerations. At the same time, it became evident that the requirement for a true zero-visibility operation (as originally envisaged in the ICAO Category definitions) had diminished, as clean-air laws had reduced the adverse effect of smoke adding to radiation fog in the worst affected areas. Improved avionics meant that the technology became cheaper to implement, and manufacturers raised the standard of the "basic" autopilot accuracy and reliability.