As an american, I always thought these were somewhere between urban myth and scare tactic... Didn't realize they actually worked!

There was a very small window in time when they could, but they're all but useless now, simply being touted as a scare tactic.

I can't find the source for the life of me; but detector van operators haven't been called to give evidence to support a prosecution in decades; and whereas TV Licensing historically haven't disclosed their fleet size or operations, their budget drops would suggest that the automotive arm that would support their detector van procurement has been dormant for years.

edit: in fact I think most of the enforcement actions have been outsourced to Capita, which is the equivalent of putting Myra Hindley in charge of a fucking nursery.