I love old maps. During my travels, old US highways and backroads are my preferred method of seeing our country, as opposed to driving interstates.
I found a US government statistic on Vehicle Miles Traveled and Miles of Public Roads that estimates the period from 1920 to 2013. Comparing 1940, year of your map, to 2013, latest year of data, is quite revealing. The chart, attached below, has three separate line graphs. The chart is a pdf file, but the following web link has statistical data:
https://www.fhwa.dot.gov/policyinformation/statistics/2013/vmt421c.cfm
I hope the attached opens for those interested. The blue line is VMT (Vehicle miles of travel) and corresponds to the scale on the right side of the chart, measured in trillions of miles. The red line is MPR (Miles of Public Road) and corresponds to the scale on the left sight of chart. The green line, that only extends back to 1975, depicts Miles of Road per lane. This information cannot be used for the comparison below, due to being incomplete.
For 1940, there were about 302,188,000 miles of vehicle travel for 3,287,000 miles of road. Dividing the first number by the second number yield a density factor of 92, travel per road. The miles are cancelled out. The density factor only has meaning when compared to other periods of time.
For 2013, there were about 2,988,323,000 miles of vehicle travel for 4,115,000 miles of road, yielding a density factor of 726.
Dividing density factor of 2013 by 1940 yields in increase of density of 7.9 times.
It must have been nice to drive in earlier days. The statistics do not indicate miles of pave versus unpaved roads, so I’m assuming the ratio of paved versus unpaved has improved over the years. Also, the Miles of Road per lane would mitigate the increase in density somewhat, if the data was available pre-1975. On the other hand, I live in Phoenix, so the more lanes per road just means drivers change can switch lanes and become more of a nuisance.
Comparing the density factor on a ten-year basis, using the beginning year of 1940 as the fixed point for comparison, yields the following increase in density per decade: 1940 = 1.0; 1950 = 1.5; 1960 = 2.2; 1970 = 3.2; 1980 = 4.3; 1990 = 6.0; 2000 = 7.6; and 2010 = 7.9.
Finally, to keep things in perspective, the US population in 1940 was 132.1 million people and estimated to be 325.7 in 2017, an increase of 2.5 times. If the density factor was based on a per capita basis, the increase in density be more understandable and wouldn’t seem so severe. But then again, I’m only concerned with my driving and how much traffic density I encounter.