Some computer scientists with whom I used to hang back in the day have
produced a marvelous little paper on a cryptographic scheme to make
traffic cameras that respect driver privacy. At a slender five and a
half pages, it's a quick read (the bloatedness of so much legal
writing is making me nostalgic) with a simple, easy-to-grasp idea.

The basic idea is to replace cameras that snap pictures of license
plates with EZ-Pass style transponders. On detecting a violation, the
traffic enforcement base station would demand that the violator's
transponder identify itself. The clever and well-known part of the
scheme is that the transponder would reply not with its actual
identity but with a pseudonym. Through some zero-knowledge proof
trickery, drivers would need to come forward, identify themselves as
being behind their pseudonyms, and pay their tickets in order to be
able to keep their transponders working.

The clever and original part of the scheme is that the transponders
would change pseudonyms rapidly -- once a second. That way, the
network of base stations couldn't be used to track a single vehicle
around, effectively keeping the network from being used for other
kinds of surveillance. The truly neat part is that it's possible to
stitch together well-understood cryptographic primitives in such a way
as to implement this rapid name-changing without weakening the
condition that violators need to pay their fines in order to keep on
driving.

One may have legitimate qualms about the non-cryptographic parts of an
automated traffic enforcement system (e.g. are the clocks properly
calibrated?) One may have legitimate qualms about whether a system is
actually implemented in a way that preserves privacy. But I now feel
more confident that such systems can be implemented in a way that
preserves privacy.

http://research.yale.edu/lawmeme/modules.php?name=News&file=article&sid=1713