We expect simple graphical features to be both suggestive of and immediately recognizable as what they represent. We can imagine some surface simulation in order to maintain the recognizable characteristics. For example, something working hard should look like it is working hard even if the exact visible motion is a fabrication to enhance recognizability.
Radio traffic emanates from New York City like lightning.
Our Java SimView was written before the swing library supported line thickness as a property. In place of this we fuzzed the location of endpoints such that thin lines assumed a thickness that increased with the number of lines displayed. The effect looked like electric arcs which is a suitable metaphor for radio communication.
See Web Traffic for a dense interpretation of resource competition in a web server. demo 
See Table-Top Augmented Reality for a spatially endowed display system.
See Perlin Noise for synthetic texture.
See Grasp It for decorated physical simulations.