Paving the Way: Traffic Flow Maps From the 1920s

Soon after the car came, inevitably, car traffic. Since the dawn of the automobile, the wide open road would become less and less wide open as private car ownership came to dominate the American transportation landscape in the early 20th century. While revolutionizing travel for many, the early decades of automobile use were fraught with problems, from alarming safety concerns for drivers and pedestrians, to road infrastructure not designed or built for this new mode of transportation (and an increasingly popular one at that). To measure the impact of this new technology and plan for the future, governments would turn to maps to try to make sense of this burgeoning new era in transportation.

Automobiles began gradually appearing on American roadways in the late-1800s, but it was in the 1920s that cars really proliferated across the country. Between 1919 and 1929, the number of passenger cars in the U.S. ballooned from 6.5 million to 23 million, quickly overwhelming local road infrastructure. In 1922 alone, the federal government completed over 10,000 miles of roadway improvement projects.

At national and state levels, the need for understanding and quantifying roadway usage would lead to ambitious mapping projects relying on manual counts of cars and trucks passing through different nodes of the highway network. Among the vast collection of road maps in the Geography and Map Division are traffic survey maps coinciding with the 1920s boom in car ownership and roadbuilding.

Surveys of traffic in the 1920s would often take the form of “flow maps,” a type of thematic map with linear symbols indicating the direction and magnitude of movement. This type of map long predates its use here; French civil engineer Charles Minard’s flow maps from the mid-19th century on such subjects as English coal exports and the movement of cereals over French waterways and railways demonstrate the longstanding utility of this map visualization.

This 1924 traffic chart of state highways of the Upper Midwest (as well as a version created the following year) is a prime example of a flow map, with thick lines of high traffic emanating from urban centers and thinning out with connections to smaller communities.

This 1927 map of New Hampshire traffic is more complex, incorporating not only car and truck counts but also indicating forecast traffic in 1931 and 1936. Combined with orange shading of the state’s most densely populated urban centers, the overall map is dizzying and difficult to fully decipher, but it speaks to the urgent intention of the map’s creators in trying to fully understand the rapidly changing nature of road usage.

Perhaps the most eye-catching 1920s traffic map in our collection is this 1921 map of the Maryland state highway system. Produced by the University of Maryland, this map contains an incredible amount of data for the time period. Beside the prominent flow lines are the average daily number of all vehicles and trucks in 1917, 1918, 1919, and 1920, taken at locations across the state.

Additionally, six polar diagrams provide a sampling of monthly traffic trends at various locations. Some distinct trends emerge here, but generally, these diagrams all show traffic peaking during the summer months, especially in August, and declining significantly during the winter months. Two line graphs on the left-hand side of the map, showing traffic volume over time by mode, speak to this unique time period: traffic for all vehicles and trucks are on the rise, while use of “Horse Drawn Vehicles” is on the decline.

Modelling car traffic today is a complex process, and yet we don’t often think about the technologies that underpin the navigation apps on our phones that help us avoid traffic jams and find the fastest route to our destination in real-time. Data on traffic conditions may be a boon for drivers, but it is absolutely essential to transportation planners scoping out the growth and maintenance of vast, multi-modal networks for getting people where they need to go.

For as sophisticated as this data collection and analysis has become, the traffic flow maps of the 1920s demonstrate how geographic transportation modeling has long predated GIS technologies. In fact, traffic mapping long predates digital methods entirely.