"Existing systems"
Trafficmaster = shite. Patchy coverage, out-of-date information abounds (I always get two different tales of the M25 from two different blue masts on my way home which I pass within 30s of each other), and their cameras aren't always sited well (M4 J10 -> J8 in both directions has sod-all coverage) - you get told you're screwed on the way to J8 eastbound AFTER you could have turned off!
This is a great idea - short-range comms between vehicles, with ad-hoc mesh networking, and you could "see" 20m ahead of you in dense traffic and know what was coming up. At the very least, it would make trafficmaster look like the shit under-invested pile of cack it is.
This new info, combined with critical mass uptake, however, would inevitably create swarming effects and sideroads would become unusable at peak times. We'd see tech-inspired group think taken to new stupid levels!
My wife and I had a great idea to combat traffic congestion the other day - filing and "drive plan", like a "flight plan", and you have to apply for a slot to use it (this would have to be quick and easy, mind). This acknowledges that we have limited throughput on the roads we have. Our current problem is we have unlimited car ownership in comparison (especially in the UK) - get a licence, and if you can afford the running costs, you're on the road. Once we acknowledge that we need to ration the roads (and not through crap road toll ideas, it's got to be something you have to book to use, not something you need to pay for but can use whenever) and work out how to do it, we can start managing congestion.
/Paris, even she has a licence... <shivers>