The title of the paper at the following link may be a little deceiving. It contains plenty of useful information about stop, arrest and detention in Alabama according to both federal and state case law:
THE LAW of TRAFFIC STOPS
The Stop, Detention, Investigation, and Arrest of a Motorist or Passenger by
Alabama Law Enforcement Authority – State and Federal Cases
Cases current through November 1, 2009
http://www.mahaneylaw.com/files/the_law_of_traffic_stops.pdf
"... This “mistake of law”- “mistake of fact” distinction is important. Under Alabama law, law
enforcement officers are presumed to know the law they are enforcing. Reviewing courts will
nearly always give great deference to a law enforcement officer’s mistaken belief of fact, but no
such deference is afforded the officer’s mistaken belief of law. Where the seizure is
accomplished under the officer’s incorrect or deficient knowledge of law, no lawful authority
exits to support the seizure..."