Automatic transition (sic!) vs manual gearbox
One glaring imperfection in the US scientist study is the setup of the driving simulator (page 3 of the PDF).
Most drivers in Europe (UK included ^^) still have a manual gearbox in their car, so it only stands to reason a European scientist will see a BIG difference between hands-free cell phone use vs using a handheld phone. With the lazy bums... drivers in the US never needing to learn how to operate a stick shift, it's no wonder their researchers can hardly find a difference between handheld and hands-free phone use!