Amazing, but true ...
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Law of the People's Republic of China on Road Traffic Safety
Chapter VII Legal Responsibility
Article 88 Punishments for violations of the law on road traffic safety include: disciplinary warning, fine, temporary suspension or revocation of motor vehicle driver's license, and detention. (emphasis added)
source -
http://bjjtgl.gov.cn/publish/portal1/tab193/
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One thing I have learned about China is that the law is always open to interpretation, presumably to serve the laudable objective of a harmonious society. Article 88 includes "detention" as an option for punishment for ANY violation. It appears to me that Chinese laws are more "mercurial" than what we might find back home.
Last Fall, in Beijing, there was an active crackdown on illegal plates (supposedly it was China-wide). To prove the point that this was a BIG no-no, the authorities somehow classified it as a
criminal offence to operate a motor vehicle with a plate that was missing, fake, copied, altered, obstructed, improperly mounted, etc. No idea how that reclassification was done, but TIC so there is always a way. Presumably, as a criminal offence, the limit of 15 days for "administrative detention" could be extended, as well as additional penalties applied (e.g., revocation of one's visa).
Here is one article that quotes the campaign's public announcement:
http://www.globaltimes.cn/content/728118.shtml
In the larger cities, such as Beijing, many of us personally know people who have been trapped in ugly situations.
I suppose one could try and argue "I had no idea", but probably not be successful. Under administrative detention, there is no court - you are held (for a maximum of 15 days) until a "disposition" is made (by the police).
If you want further information on police powers, look into the current public debates on "black jails" and the "re-education" system. People have been held for years, without charge, for what appears to be the most inconsequential "infractions." The new government has promised a comprehensive review of this policy of detention.
Bottom line ... yes, you can be tossed in the clink for fake plates. Will you? It probably depends on whether the "investigating officer" has a daughter who was wined & dined by a foreigner.