1948 would have been carrying 1,200 trucks per day and would have been subjected daily to 91 axle loads of 18,000 pounds or more, 40 of which would have weighed 20,000 pounds or more and 17 of which would have weighed 22,000 pounds or more. Such frequencies of heavy concentrated loads as these produce severe pavement strains and frequently cause a pumping action at the joints of concrete pavements which removes support from the pavements and ultimately causes their failure.
REGIONAL VARIATIONS
The example given in the preceding paragraph is based upon average conditions throughout the United States. In some regions the situation is much worse, as legal provisions and enforcement practices result in larger differences in the frequencies of heavy gross weights and axle loads. In the Middle Atlantic region, for example, the hypothetical road carrying 1,200 trucks per day in 1948 would have been subjected daily, on the average, to 182 axle loads of 18,000 pounds or more, 112 of which would have weighed 20,000 pounds or more and 60 of which would have weighed 22,000 pounds or more. Table 9 shows the frequency of heavy axle loads and table 10, the frequency of heavy gross loads by census regions for the year 1947. A comparison of these tables shows that it is not in the regions with the highest frequency of heavy gross loads that the greater frequencies of heavy axle loads occur. Phe Pacific region, for example, has the
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