David Allen has published a paper on his website called "Making Good Use of Car Time" describing all the things you can do while driving.
I think it is worth discussing the idea that multitasking while driving may increase your risk of being in an accident. This has been a somewhat controversial area, especially regarding cell phone use, but the facts are piling up. There is good evidence that use of a cell phone while driving slows reaction time and increases accident risk a significant amount-- to the same degree as being intoxicated (at the common legal DWI limit of 0.08% blood alcohol content). This applies to hands-free devices as well as hand-held phones.
Obviously, being in an accident, especially one with injuries, is no way to increase productivity. And I doubt Allen would ever condone drinking and driving to enhance productivity (no waiting for a cab from the bar), so I am skeptical of his recommendations here.
I am not trying to troll here. I am a big GTD and DA fan. This just seem odd to me. When I am in the context of the driver's seat of a 1.5 ton guided missle, there is only one thing on my next actions list. Let's keep our focus on the road and stay safe.