The WTO Secretariat has issued an interesting report on notified quantitative restrictions:
1.1. This report has been prepared at request of the Chairperson of the Committee on Market Access with a view to providing Members with factual information on the notifications of quantitative restrictions (QRs), which have been received as of 19 May 2015 pursuant to the "Decision on Notification Procedures for Quantitative Restrictions" (hereinafter the QR Decision). ...
...
2.2. In May 2014, and as mandated by paragraph 4 of the QR Decision, the WTO launched a database on Quantitative Restrictions (hereinafter the QR database), which compiles all notifications submitted by Members under the QR Decision. ...
...
3.10. Paragraph 2(v) of the QR Decision requires Members to provide "an indication of the grounds and WTO justification for the measures maintained […] and the precise WTO provisions". Although Members have invoked one WTO provision in 87.9% of the QRs notified, they have mentioned more than one justification in 5.3% of them, whereas nine Members did not provide any information for 53 QRs (6.8%).
3.11. The GATT 1994 has been the most frequently cited Agreement (more than 90% of the QRs), and in particular Article XX (General exceptions), which has been referred to in 593 QRs. Particularly relevant is the reference to paragraph b of Article XX (measures "necessary to protect human, animal or plant life or health"), which accounts for 311 citations. Article XXI:b, concerning exceptions necessary for the protection of a Member's national security interests, has also been included as justification in 89 cases.
What are all these measures for which Members are citing GATT Article XXI? Some enterprising grad student should take a look!