Driving in the United States and other countries, one could make an educated guess about the relative importance of a road by looking at its route marker. See Figure 8.1 for a collection of route markers from different ranks of interstates, U.S. Highways, state highways, and county roads. Roads of higher ranks tend to be wider, faster, and carry more traffic. Taking Minnesota as an example, there are approximately 19,300 kilometers (12,000 miles) of Interstates and state highways (9% of the total road length) as of 2005, which account for about 60% of the total 87 billion annual vehicle-kilometers (54 billion vehicle-miles) traveled in this state (Minnesota Department of Transportation, 2005a).
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