Big Brother in the Odometer

How does the government know how much I’m driving?

One car … two cars … three …

Americans have reduced the number of miles they drive each month since November, according to a report released Wednesday from the Federal Highway Administration. Americans drove 53.2 billion miles fewer than they did over the same eight-month period a year ago. How does the U.S. government know how much time I’m spending in my car?

Four thousand little sensors and a bit of algebra. States install traffic sensors on urban and rural roads and interstate highways to count cars and, sometimes, to determine their size. (You may have noticed their lane-wide rectangular outlines embedded in the pavement.) Some sensors detect vehicles by their weight. Others detect a car’s metal underbelly with an electromagnetic field. At the end of every month, the states pass the car-counting data to the feds, who plug them into a formula to estimate how many miles Americans drive each month.

A pair of sensors placed near each other can sometimes be used to determine the size of the cars that pass over them. First, they determine a vehicle’s speed by measuring the time it takes to go from one sensor to the next. Then each sensor can measure the time that elapses between each pair of wheels on the same car. Together, those data can be used to figure out the distance between the vehicle’s axles and what type of car it is.

State highway agencies had been collecting this data for decades before the Federal Highway Administration even existed. In fact, the technology for sensing traffic dates to the 1920s. Highway engineers used pneumatic tubes that shot a burst of air and activated an electrical signal every time a car drove over them, much like today’s weight-based devices.

The modern versions of these machines aren’t infallible. If two motorcycles pass over a sensor at the same time, for example, their combined weight and size might be confused for one small sedan. Sensors are supposed to be replaced after road construction, but workers sometimes unwittingly destroy sensors, creating a black hole for car stats. Big accidents can also damage the road and wipe out the sensor.

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Explainer thanks Doug Hecox * from the Federal Highway Administration and Dan Middleton from the Texas Transportation Institute.

Correction, Aug. 18, 2008: The name was originally misspelled.