Source material is a more defined list of facts for the lawyer, the urban planner, the chef and the realtor. But you are an artist and everything is source material. In fact the one thing you don't jot down may be the very key to the next phase of your work.
With notetaking/sketches come ideas. Your work evolves...but what happens when studio time is so scarce that ideas and sketches stack up? You want to see your ideas through, but before you can work out an image, you bump into new ideas: architecture, mapping, strength of the common man... All of the sketches in all of the sketch books become homework waiting to happen, a philosophical tsunami of unrealized ponderings.
I work as a scenic painter for film in NY. I live in Boston. I drive a lot. The driving is tough because it is lots of thinking and no drawing. Scenic painting is great in that I learn many things and many materials. I am basically a glorified house painter. My hands are often full. Days are long, work is grueling and coffee breaks are sacred.
All of this cuts back on my drawing. The Rhodia 5x5 pocket sketch book w/graph lines has become my favorite. It is discreet. Jotting ideas down at work doesn't involve a wood shim or sandpaper, and drawing people on the subway is less of an ordeal. I appreciate that.
The trick - take in the information. Let the ideas simmer. Show up at the studio with a solid plan. If I don't have time to put together a full body of work, or even a series, make one solid image...