Data from United States Department of Transportation, Federal Highway Administration.
The above map represents four ways to classify and symbolize transport-related information:
- Nominal classification. A simple distinction is made between highways that have the status of interstate maintained by the Federal government and US and State Highways of the same class but maintained by state or local governments. 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 the congestion level), is classified into 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.