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 <answer>2006</answer>, in practice, these days most vehicles are exempt from the circulation restrictions as long as they pass regular emissions tests.
When were new emisson standards enacted?
2006