I want to know the answer to the following question:
How can cars get exempt from the restrictions?
All the background I'm provided with is that "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.". Is that enough to answer the question?