The Symbolization of Transport Features

• Nominal classification. A simple distinction is made between Interstate highways that are toll and those that are regular (non-toll). There is no order in this classification since each class is simply different from the other.
• Ordinal classification. A distinction is made between major roads according to their capacity (low, average, high). In this case there is an implied order in the classification as some road segments have a higher level of importance than others.
• Interval classification. A quantitative variable, the volume to capacity ratio (a good indicator of congestion), is classified in distinct and clearly bounded categories.
• Proportional symbols. 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.