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Big Brother in the Odometer How does the government know how much I'm driving?

By Derek ThompsonPosted Friday, Aug. 15, 2008, at 6:48 PM ET

Cars in trafficAmericans 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.

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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.

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Derek Thompson is a Slate intern.
Photograph of cars in traffic by Daniel T. Yara.
COMMENTS

Remarks from the Fray:

I used to work for a state DOT, and was very amused to find out that it considered truckers who fueled up just shy of the state line (where gas taxes were $0.13 less per gallon) to be lawbreakers. Not in the sense that the Highway Patrol could pull them and write a ticket, but they considered the lost revenue to be a form of tax evasion and were constantly trying to figure out how to resolve this issue.

Part of the problem stemmed from the fact that my state maintained a much higher percentage of road miles than the other state, which relied on counties to maintain all those expensive, out of the way roads that are little used compared to interstates. For whatever reason, there is a huge discrepancy.

DOTs nationwide have discussed forcing automakers to equip cars with tracking devices for calculating road miles driven, though naturally this has immediately raised privacy concerns and hasn't really been taken seriously.

DOTs in general see gas taxes as much too crude a measure, because of differing fuel efficiencies and the fact that a mile driven on a lightly used suburban road during off-peak hours costs the state a lot less than a mile driven at rush hour on an intra-city corridor ("Interstate" highways being used by local commuters).

They bitch and bitch that Priuses, etc. are costing them money because they are too efficient. Naturally one wonders why they don't tell the legislature it's either higher gas taxes or shittier roads, because with fuel efficient cars becoming more popular and construction costs soaring due to inflation (China buying more steel and concrete, etc.), gas tax revenue is not keeping up, and something's got to give.

Anyway, raising our already high-ish gas taxes is evidently not an option (even though our state's economy keeps chugging despite the recent gas price surge). I don't get it, but I'm worried about what the legislature is going to do to get road money, and I'm thinking it's not going to be pretty or equitable.

--Radiotone

(To reply, click here.)

(8/17)

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