A study sponsored by Navteq argues that having a GPS navigation system with real-time traffic data lets drivers spend less time behind the wheel — between two and a half to four days a year, depending on the country — and produce around 20 percent less CO2. The problem with these reported results, which are based on a study conducted in two German cities, is that they compare drivers with traffic-enabled navigation systems to drivers without any navigation system. But the study also included drivers who used navigation systems without real-time traffic data. Is it the real-time traffic data that offers an advantage, or just having a GPS in the car? How much of the advantage comes from the traffic data, and how much from the navigation? Navteq’s press release doesn’t say. That omission is worth paying attention to. Via Engadget.