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of cities. Most of the 31,831 rural miles of the system is entirely free of such traffic control.

Conditions in cities

In cities the condition is very different. On the 5,969 miles in urban places there are 9,036 installations of such signs and signals, an average of 3 for every 2 miles. In the larger cities—those over 5,000 population—they average 2 to the mile. In the largest cities they are, of course, of still more frequent occurrence.

Stopping necessitated by these signals accounts in part for the very slow movement of traffic that is found to exist on urban sections of the system. In larger part, perhaps, the slow speed is occasioned by the many other obstacles to movement encountered. Among these are many unsignalized cross streets, jay-walking pedestrians, the midblock halting of vehicles to load or unload, the maneuvering of vehicles into and out of parking spaces, and the double parking of vehicles.

Free flow of traffic and turning movements on urban portions of the interstate system are often hampered by traffic lights, narrow streets, parking, and entrances to gasoline stations and other business establishments, as shown here on US Route 1 in Virginia. Improvement to interstate system standards would save 21 billion vehicle-minutes—almost 40,000 vears—annually.

Speed of movement in cities

All of these causes together result, as shown by actual running tests made in all cities of 5,000 or more population, in an average speed of movement on sections of the system in cities of these sizes of only 18.1 miles per hour during the hours of peak traffic and 23.5 miles per hour during the off-peak hours. There is almost no variation from these averages in cities, from the smallest to the largest.