I think that police, prosecutors, and insurance adjusters tend to like technical means of detecting and quantifying violations of laws or standards, because these means reduce the amount of discretion exercised by the enforcers of the rules and thus the amount of post-hoc argument over how that discretion was exercised. However, the laws on the books assume the exercise of discretion. Changing the amount of slop in enforcement decisions without changing the standards seems a dangerous business to me. The biggest RISK of black-boxes for automobiles is that they'll enable strict enforcement of the wrong set of standards. California has several special rules intended to fuzz traffic enforcement in favor of putative violators.
Local cops can use radar only after special formalities to justify the limits they're enforcing.