Query: When were new emisson standards enacted?
Context: There is an environmental program, called Hoy No Circula ("Today Does Not Run", or "One Day without a Car"), whereby vehicles that have not passed emissions testing are restricted from circulating on certain days according to the ending digit of their license plates; this in an attempt to cut down on pollution and traffic congestion. While in 2003, the program still restricted 40% of vehicles in the metropolitan area, with the adoption of stricter emissions standards in 2001 and 2006, in practice, these days most vehicles are exempt from the circulation restrictions as long as they pass regular emissions tests.