Out of sight...

The type of short range direct communication envisioned for vehicle-to-vehicle cooperative systems is prone quality degradation due to to non-line of sight (NLOS) conditions. This is bad because there is often a correlation between radio NLOS situations and dangerous traffic situations, for example collision avoidance at an intersection where approaching vehicles can't see each other because of a building.

An Australian company, Cohda Wireless, has received a lot of press recently regarding their Dedicated Short Range Communication (DSRC) technology which they claim improves communication quality in NLOS conditions.

From Cohdas website:

"Cohda Wireless has developed unique receive-side signal processing for the outdoor, mobile automotive market and instantiated this in radios that dramatically outperform all radios available in the world today. We leverage this unparalleled knowledge of how radios work in real-life to produce Connected Vehicle and Safe Vehicle applications that work in more places, more of the time."

A quick scholar search turns up a number of patents and a few publications. Cohda also states that they have done "more than 700 DSRC trials", whitepapers are available on request via their website.