Proportional symbols. The size of a symbol is a function of a quantitative variable. Thus, the radius of a circle or the thickness of a line can be set according to a variable. For transportation systems, proportional symbols are particularly important to express flows in a network or at terminals. On Figure 4.12, the size of a symbol, road segments in this case, is a direct function of a quantitative variable. The higher the traffic, the thicker the line segment.
Labeling. Involves the positioning of descriptive text over specific geographical features. The labeling process which is particular to transportation mainly concerns assigning identification symbols (dominantly numbers) to road segments.