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Does Your Car Have a Black Box?Event data recorders know when you've been speeding, and when you've hit the brake.

By Paul Boutin
Posted Wednesday, Aug. 20, 2003, at 12:25 PM ET

Listen to Paul Boutin discuss this topic on NPR's Day to Day.

Illustration by Mark Alan Stamaty

After South Dakota Rep. and former Gov. Bill Janklow, R-S.D., collided with a motorcycle last weekend, a state prosecutor said investigators believed Janklow may have run a stop sign, causing the crash in which the motorcyclist died.*

The Associated Press reported that according to a South Dakota highway patrol officer, "Janklow's 1995 Cadillac has a black box, which records information such as how fast the car was going and whether the brakes were applied." How many cars have such black boxes, and what information do they record?

Black boxes—event data recorders like the ones found in airliners—are increasingly common in automobiles and vary from one type of car to another. But cars with airbags have long had onboard computers with the sensors and software necessary to determine within 1/100 of a second that you're in a crash; that's how cars know when to deploy the bags. These computers, called sensing and diagnostic modules, are located inside the transmission hump, behind the dashboard, or under the seat, and constantly collect and process data on the car's acceleration or deceleration. Airbag-equipped cars made by General Motors (which owns Cadillac) have had SDMs since 1974.

Beginning in the 1999 model year, though, GM upgraded SDMs to include an event data recorder. The newer SDMs track the car's speed (from the speedometer), engine RPM, the exact position of the gas pedal, and whether or not the brake pedal was pressed, among other statistics. The SDM keeps the previous five seconds' worth of this data in its onboard memory and, if the airbags are deployed, saves the most recent five seconds as a snapshot of events leading up to a possible collision. Ford and Isuzu added similar features to some models in this decade. Santa Barbara-based Vetronix sells a $2,500 "crash data recovery" gadget that will download the logs from these computers (the company lists what years and models it works with, and what data is recoverable).

Auto engineers designed and installed event-logging SDMs to study accidents and improve their cars' safety, but the data from the boxes has also proven admissible in court. This June, a Florida driver was sentenced to 30 years in prison based on the data in his car's SDM, which showed him to be barreling down a suburban street at 114 mph seconds before he struck and killed two teenagers in another car.

But a GM spokesperson says Janklow's 1995 Cadillac DeVille, which deployed both its airbags in the crash, would have an older SDM without the event data recorder. Prosecutors won't be able to chart the congressman's speed, or tell how he'd used the gas and brake pedals, in the seconds prior to the collision.

Next question?

Explainer thanks Bill Kemp of General Motors.

Correction, Aug. 20, 2003: This article originally stated that Janklow struck and killed a motorcyclist. In fact, the motorcyclist died after hitting the rear driver-side door of Janklow's car, and prosecutors say Janklow may have run a stop sign at the intersection just before the crash.(Return to the corrected sentence.)

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Paul Boutin is a writer living in San Francisco.
Illustration by Mark Alan Stamaty.
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