{ subscribe_url:'/share/sites/library-of-congress-blogs/geography-and-maps.php' }

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.

Map of traffic flow lines across New York, including thinck lines for major highways (in blue) on white background.

Traffic census map: showing the maximum traffic over state highways for a period of 12 hours during the month of August 1920: [state of New York]. Map by New York State Commission of Highways, 1921. Geography and Map Division.

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.

Blueprint flow map of Upper Midwest states with thicker flow lines near major cities, smaller lines near smaller communities.

1925 traffic chart, state highways systems: [North Central States]. Map by Bureau of Public Roads, District No. 4, 1924. Geography and Map Division.

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.

Flow map of traffic distribution on New Hampshire highways, with red and blue line widths indicating traffic and projected traffic.

Highway traffic survey of the state of New Hampshire: motor vehicle traffic on the state highway system with traffic classification and population distribution. Map by U.S. Geological Survey, 1927. Geography and Map Division.

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.

Blueprint flow map of Maryland traffic with thick white lines emanating from ajor cities, thinner lines to smaller cities, and polar charts and line graphs showing traffic data.

Traffic map of Maryland: showing distribution of traffic over the state highway system. Map by University of Maryland, College Park, Engineering Experiment Station, 1921. Geography and Map Division.

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.

Detailed view of Maryland traffic blueprint map, focused on thick traffic lines between Washington and Baltimore, enumerated traffic counts, and polar chart of monthly travel.

Detail of Traffic map of Maryland: showing distribution of traffic over the state highway system. Map by University of Maryland, College Park, Engineering Experiment Station, 1921. Geography and Map Division.

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.

Sanborn Fire Insurance Maps – An Orientation and New GIS Tools

Please join us for the second session in a new series of virtual orientations for the Geography and Map Division, focusing on our collection of fire insurance maps! Date: Tuesday, August 9th, 2022 Time: 3:00-4:00 pm (Eastern) Location: Zoom Register Here Reference librarians Amelia Raines and Julie Stoner will present an introduction to the fire insurance maps housed […]

Mapping the Gangs of Chicago

When Prohibition became law across the United States in 1920, legitimate businesses were no longer allowed to serve alcohol, paving the way for illegal speakeasies and related underground businesses. In Chicago, this meant that criminals like Al Capone and Johnny Torrio fought for control of illegal alcohol distribution within the city, sparking an infamous decade […]

What Goes Up Must Come Down: A brief history of New York City’s elevated rail and subway lines

This is a guest post by Sonia Kahn, Library Technician in the Geography and Map Division. On a recent trip to New York City, I frequently found myself in the underbelly of the city, submerged below the hustle and bustle as I was transported up and down Manhattan. I couldn’t help but notice while I was visiting […]

Out and About in a Provincial Empire of Fascism

It is only natural that Rome, by reputation being the “Eternal City,” has evolved over its roughly twenty-seven-hundred-year existence. Even the briefest visitor would be hard-pressed to overlook the glut of imperial detritus, some ancient, most merely old, and some modern. All the relics from the latest phase seemingly appertain to the ill-fated regime of […]

Louise E. Jefferson – A Hidden African American Cartographer

This is a guest post by Iris Taylor, a senior cataloging specialist in the Geography and Map Division. It is a common belief that you can acquire inspiration from a variety of people, places, or things. Seanna Tsung, a Library of Congress staff member, recently uncovered a unique collection of maps in the Geography and […]

Visualizing Injustice: Early NAACP Cartographers and Racial Inequality in America

No good result can come from any investigation which refuses to consider the facts. A conclusion that is based upon a presumption, instead of the best evidence, is unworthy of a moments consideration.                       –Ida B. Wells, 1901 The use of cartography to highlight economic and […]