I'm sure the black boxes are all different but my son's one definitely did know the speed of the actual road (like a sat nav would). One plus of this is that, in the event of your car being stolen, the insurance company can use the black box as a tracker. Also some companies will ring the...
The cheapest my son could get last year at age 17 was with Ingenie with a black box, it cost about £1,200 but there was money back each quarter if his driving was up to scratch - he got about £150 back during the year. He has just renewed with Admiral as this was cheaper and also no black box...