Supreme Court Illinois v. Caballes, 543 U.S. 405
Page 11
The permissible scope of a routine traffic stop is clear: A
motorist reasonably expects that she will be detained briefly
while the police officer asks for her name, checks her
driver’s license, and properly documents her alleged
violation. She also reasonably expects that she will then be
allowed to resume her journey, without being subjected to an
invasive, unrelated drug investigation.
The deployment of drug-detection dogs cannot be justified
by the legitimate investigative needs of a routine traffic stop.
Page 24
at
the sole discretion of a street-level police officer acting on his
own initiative, is precisely the kind of “standardless and
unconstrained discretion” that this Court has previously
condemned. Edmond, 531 U.S. at 39, quoting Prouse, 440
U.S. at 661.
In sum, the use of a drug-detection dog at a routine traffic
stop, even where the stop itself is based on probable cause,
impermissibly alters the nature and quality of the stop.
Given the absence of any logical relationship between traffic
violations and drug offenses, the transformation of a traffic
stop into a drug investigation is simply arbitrary. Absent at
least reasonable suspicion, such a drug investigation cannot
withstand Fourth Amendment scrutiny.
Page 36
Drug-detection dogs
have proved to be less reliable and disciplined than the Court
thought in Place, where it assumed that canine sniffs
categorically “do[] not expose noncontraband items.”‘ 462
U.S. at 707. In fact, research shows that the reliability of
dogs varies widely based on several factors, including the
degree of its underlying training and the skill and attitude of
its handler. Another significant factor is the randomness of
the inquiry, with false alerts being much more likely in cases
in which individualized suspicion is absent.
Here is the PDF
http://www.aclu.org/files/FilesPDFs/caballes.pdf