I often hear writers say they feel so unproductive if they haven't set words to paper/screen on any given day. —That somehow the day is lost without that activity marking some measure of writerly success. Certainly little is actually written unless it's set down in sentences and turns of phrase, but just because words don't necessarily make it to the page doesn't mean that a writer isn't busy at work.
I work quite a bit on long car rides, even when I'm the one behind the wheel.
Don't get me wrong, I've not actually got my laptop propped on the dashboard, but I do keep a little notebook in the center console. It's full of cryptic notes I can barely decipher—single scribbled words that slant across the page.
The real writing happens in my head. Scene sketches, imagined conversations, character development, and every once in a while I get some major plot points worked out. And whether or not it all ends up written out at the end of the day doesn't discredit the fact that I accomplished some serious work.
It was Rudolph Erich Rascoe who said, tongue in cheek, "What no wife of a writer (or husband in my case) can understand, is that a writer is working when he's staring out the window."
Quite frankly, I think that's a good thing to keep in mind. A working writer might not appear to be working, but don't interrupt her if she is staring at a blank wall smiling faintly, or conversely, wearing a fiercely determined expression. —She's most likely building a story and if you're not careful you could alter the plot. Or, depending on the frequency of interruptions, the security of your own existence.