is gonna get anywhere anytime soon... it's nearly worthless until every car on the road has it.. which will take a LONG time.. even getting to something like 90%+ v2v-enabled will take decades.
The benefits start accruing once 10 percent of the vehicles on the road have it. You don't need 90%. You don't even need 30%.
As you rush headlong into a fogged in traffic jam, there is a good chance that at least one vehicle in that jam will this technology and warn your car well ahead of time, so you can slow down (also slowing those behind you). You don't need every car to have this. Similarly, in-road transmitters can warn just enough new cars of trouble ahead to slow an entire stream of traffic.
Sure, not ALL of the capabilities of V2V will be available immediately, but plenty of them will work even with a small percentage of participants.
That being said, development of these systems is far from complete, and shifting them to new frequencies is really a last minute decision. There is no real reason that 5GHZ is ideal for this V2V use, and something much higher up in the spectrum might actually work just as well, if not better.