Scott C. Parker/YouTube screenshot by Chris Matyszczyk/CNET

Knowing you're right doesn't always help.

Convincing others of your rightness, you see, can at times be impossible. All you're left with is your conviction.

David Erasmus reportedly had the conviction that he drove through the village of Hendy in Wales within the speed limit of 30 mph. This conviction clashed with the fact that he'd received a speeding ticket claiming he'd been going 36 mph.

It's hard to fight speeding tickets. Erasmus, though, is a retired engineer, so he racked his brains as to how this mysterious ticket might have been generated.

As the Telegraph reports, he used his eye for detail and began to wonder whether someone in authority had an exaggerated sense of length.

There are white road markings that the speed camera uses to judge car speeds. Erasmus wondered if those markings were a little closer together than they should have been.

Did he use a fancy gadget, like the one golfers use to measure distance, to find out? Not quite. He got down on his hands and his knees, clutching a tape measure in the former.

The lines were, he concluded, three inches shorter than they should have been. So he went off to court and showed his evidence to a judge.

The judge took one look and expunged the ticket.

The speed camera had been in place for 12 years. How many more drivers might have been unfairly measured? How many might have suffered fines or even suspended licenses because of faulty technology?

GoSafe, the company responsible for the cameras, issued a statement saying it would look into Erasmus's tape-measuring.

The local council, too, told the Telegraph that it would pull out its own measuring equipment. What if it attempted to use something more sophisticated than a tape measure?

Whom can we trust to referee whether three inches really are three inches?