Specs are only Home Office approved for detecting speeding in ONE lane. If you find yourself creeping ever so slightly over the speed limit then change lanes. As long as you leave the SPECS section in a different lane to the one you entered then they cannot 'do you'.

And to all those about to be sanctimonious about speeding I'd rather have someone heading towards me at 80mph watching what they're going, than someone doing 70mph and staring at their speedo!