@The black helo brigade
"in the case of serious accidents - this already happens"
With a lot of guesswork. Wouldn't you rather have better info?
"Tell me, what exactly stops this device calling home twice a minute?"
Er - the fact that (a) the hardware needs to be there, (b) something needs to be receiving it, and (c) you're not important?
Bluetooth and wifi are both non-starters, unless the gov is considering putting a repeater in every lamp-post. (Which is technically possible, I guess, but if you think the gov is going to be doing that simply in order to get you for speeding, I suggest seeing your shrink ASAP.) The only possible way is using mobile phone networks, and giving your car its own SIM. Which might work, except that at yearly stats of around 250 hours of car driving per person per year, a message every 30s, and messages costing a penny a time, that's £18bn a year to run it. Nuttier gov schemes have been proposed, but not many, and not outside the MoD. Again, if you reckon "They" will stump that up to nick you for speeding, consult your shrink.
Never mind bridges - if you reckon this is a step on the way to tracking your every move, I've got a nice shiny UFO to sell you, fresh out of the ground at Roswell. Of course I can't post it on eBay, because They are watching there. Except They might be watching here too. Oh no, quick, hide!