Here's an idea I'd like to try to help myself and other artists who want to sketch more quickly, for
example for sketching people passing by during "urban sketching".
1) Take lots of photos of people in an environment similar to the one you'll be drawing in.
2) Edge-detect them. The results would look rather like this.
3) Edit away parts of the images that aren't relevant to the step below.
4) Feed the results into a program that searches for line shapes that are common to a lot of the images. For example, the rough "K"-shape made by the backs of the legs of someone standing at ease at a counter, one leg straight and one bent. The straight leg makes the straight part of the K, and the bent leg the "<" part.
5) Process the resulting list of line shapes so that they grab the attention as much as possible. Perhaps by increasing their contrast, orcolouring them yellow or red.
6) Train the artist on the results.
The idea is to equip the artist's brain with a repertoire of shapes that often occur in the scenes being drawn. This could be useful in at least two ways. First, it might help them remember the scene. When trying to memorise what I'm seeing so that I can draw it later, I find that I consciously search for such shapes and then try to vary or "modulate" them so that they match the appropriate part of the scene exactly.
Second, it might help them draw the scene more quickly, if the artist has practised drawing these shapes as well as memorising them.
I suspect that most artists do build up a reportoire of such shapes without conscious effort, as they continue to draw. But acquiring them in that way is haphazard: surely we can make doing so more efficient.
What I'm basing my idea on is the well-known psychological phenomenon of chunking. After writing the first version of this post, I discovered that there has indeed been research into drawing and chunking. One of the papers I found was "Graphical Production of Complex Abstract Diagrams: Drawing Out Chunks and Schemas" by Unaizah Obaidellah and Peter Cheng. They asked subjects to draw a variant of the Rey-Osterrieth Complex Figure: by tracing, by copying, by drawing immediately from memory, and by drawing from memory after a delay. All their subjects appeared to organise the figure being drawn into chunks for all the drawing tasks, even tracing.