In a downloadable 65-page pdf report, the NHTSA not only rates the current systems, but also offers recommendations to Congress on how to deal with the problem of backup deaths that they aren't solving. With more and more vehicles on the road, more two-income families, and trucks and SUVs accounting for a larger portion of vehicle sales over the past decade, the number of small children hit or killed while backing up has actually grown despite the availability of these technologies. Back-up incidents account for 183 deaths and 7,000 injuries every year.
As summarized by The Driving Woman, there were a few major discoveries. The bumper dot sonar sensors don't work as well as the cameras. None of the OEM or aftermarket sensors were consistent at picking up the test objects. Further reducing their effectiveness are the false positives they emit, making drivers more likely to ignore legitimate warnings when they do occur. Although better than the sonar systems, cameras were certainly not foolproof. Rain, snow or bright sunlight significantly reduced their effectiveness, for example.
The overall conclusion was that these technologies are flawed enough that very few deaths or injuries will likely be avoided as a result of their use. The report calls them expensive, easily damaged, and effectively shows they don't really work. Not a good combination.
[Source: The Driving Woman]