Car air bag "black box" used as evidence

Jul 7 15:42 UTC 2003

I figured it was only a matter of time until this happened.  I can't say
I can think of anything really wrong with it right now.


Evidence from a car air bag's electronic data recorder played a vital
part in sending a drunk driver who killed two girls to prison for 30 years.

Edwin Matos, 47, was drunk and speeding in a suburb near Fort
Lauderdale, Florida when his Matos' 2002 Pontiac Grand Am ploughed into
a car driven by a teenage girl who pulled out of a driveway into his path.

Jamie Maier, 16, and her friend, Paige Kupperman, 17, were both killed
instantly by the resulting crash last August. Matos survived.

In court last month, Matos's lawyers claimed he was travelling 60 mph in
a 30 mph zone. But data from the air bag's electronic data recorder
showed he was travelling at 114 mph seconds before the crash.

This evidence contributed to Circuit Judge James Cohn decision to
sentence Matos to a maximum 30 years imprisonment when he was convicted
on two counts of manslaughter for his crimes. The Judge also ordered
Matos to pay more than $17,000 to the two girl's families for funeral
expenses, AP reports.

Data recorders in air bags can record a car's speed and deceleration and
other data such as the pressure on a brake pedal at the time of a crash.

An estimated 10 million vehicles in the US are fitted with such
recorders, which vehicle manufacturers began putting in vehicles in the
1990s to test air bag performance.

In recent year court prosecutors have begun using information from these
data recorders, the existence of which most drivers will be unaware, in
criminal prosecutions, AP reports.

(Source: http://www.theregister.co.uk/content/55/31564.html)

response 2 of 36:

Jul 8 02:08 UTC 2003

I agree with Russ on this one.  I have real concerns about admitting those
readings as evidence, given that the equipment wasn't designed for that level
of reliability and accuracy, and we don't know what its failure modes are.
Or how reliable it is.

response 4 of 36:

Jul 8 03:04 UTC 2003

The dead bodies cushioned the blow.  Actually, one would have to know the
details of what physically happened in the accident to know whether or not
survival of such a crash was realistic... but I think it's certainly possible.

Black box technology has been well developed over the years both for use in
aircraft and, more recently, in automobiles.  Black boxes equipped in race
vehicles provide, for the most part, extremely accurate readings of things
like speed, G-force, tire pressure, and so on.  They have to--the amount of
money tied up in what the black boxes say is actually quite high.  I have no
doubt that solidly developed black boxes can provide strong, accurate pictures
of what was happening to a car prior to a crash.

However, lie detectors do a good job of telling whether or not people lie,
too, and black boxes need to be well proven before they should be used as
court evidence.  I have no idea what the pedigree of the black box in the car
involved was.  Any word on whether or not it was well-proven?

response 6 of 36:

Jul 8 06:46 UTC 2003

An inertial accelerometer could be used to measure the speed - they've almost
certainly got one in there just to help figure out when the airbag ought to
be triggered.

And hey, "airbags save lives", maybe that's how the drunk lived through the
crash.  Anyway, his real crime was driving drunk, and a simple & reliable test
for blood alcohol would prove that.

response 8 of 36:

Jul 8 14:13 UTC 2003

Re #1: The wheelspin argument hadn't occurred to me because I assumed
the 114 mph figure came from extrapolating from accellerometer readings,
since the airbag control unit would have to contain an accellerometer to
begin with.

What I really don't understand is why it matters whether he was going 60
mph or 114 mph in a 30 mph zone.  Either way it's a ridiculous speed to
be going there.

Re #3: How likely he would be to survive a crash at 114 mph depends
entirely on what he hit.  It's not the speed that kills you, it's the
deceleration.

Re #7: A friend of mine has a neat toy...a small Gunn diode transmitter
and feedhorn connected to a 9V battery and a push button.  It's amusing
to hit the button as someone streaks by you on the freeway and watch
them hit the brakes as their radar detector goes nuts.

response 10 of 36:

Jul 8 17:35 UTC 2003

I did some web searching on how the "black box" works. It has an
accelerometer that triggers the airbags and also writes to memory data for
car speed, engine speed, brake actions, and other things. However all of
these data are from 5 second before the activation of the recording. A lot
can change in 5 seconds. What I was unable to find was how the vehicle
speed is sensed. It is not by means of the accelerometer, as far as I can
tell. 

response 12 of 36:

Jul 9 02:12 UTC 2003

Re #4:  I doubt very much that you will find significant commonalities
between air-transport "black boxes" (which are actually painted orange)
and the automotive devices which bear the same label.  They are designed
for completely different purposes, and are vetted to radically different
standards (if the latter are vetted to any standard at all).

Re #5:  Given that many of the vehicles so equipped probably do not
have the per-wheel sensors used by antilock brakes, the only thing that
the designer of a generic "black box" could use is the vehicle speed
sensor at the transmission output shaft.  Wheel spin can make the
reading of such a sensor be radically wrong.

Re #6:  Sorry, Scott, but you haven't given that enough thought.  The
accelerations of interest to an air-bag trigger accelerometer are
orders of magnitude larger than the rates at which a car can accelerate
under its own power, and the quantization error and zero-point drift
of the output would make it useless to try to integrate the value to
measure speed.

Re #8:  Well, yes, 60 MPH is a ridiculous speed to be going in a
residential area.  The real question is this:  is it just to give one
drunk 30 years for killing two girls, and another one 10 years, because
one of them had a number in an EEPROM and the other didn't?  Somehow I
do not feel safer as a member of the public if chronic drunks start
driving older cars to avoid electronic finks and harsher sentences.
(Hmmm... who just replaced an old Lincoln?  With what?)

Re #9:  The reliability of most other circumstantial evidence does
not depend on un-vetted software taking irreproducible measurements,
and it's all but certain that the defense didn't have the money to
subpoena the code and hire special masters to go over it for bugs.

For more worrisome things, see the items which follow.

response 14 of 36:

Jul 9 06:31 UTC 2003

Russ, you're making assumptions about the capabilities of accelerometers. 
You're also making assumptions about what standards are set for airbag
systems... unless you've actually got some airbag design experience your
assumptions aren't any more valuable than mine.

response 16 of 36:

Jul 10 04:02 UTC 2003

Re #14:  An assumption borne of broad experience in the industry, Scott.
An engineering manager will try to squeeze out fractional cent savings
per unit, because a tenth of a cent over the annual volume of a single
car platform is several times his bonus.  This means getting rid of
any capabilities that aren't absolutely necessary for the designated
purpose of the system; if you need to trigger the airbag between 5.0
and 5.3 G of longitudinal deceleration, you can bet that the system
will not be designed to sense and integrate 0.01-0.1 G accelerations
and may very well not be accurate enough to do anything so sensitive.
Placement of sensors in vibration-prone areas may degrade accuracy further.

If you want to hear an interesting story about eking out cost savings in
automotive, pony up a beer and ask me about the Ford Corporate Switch.

response 18 of 36:

Jul 10 13:15 UTC 2003

I've always relied on fractional-cent savings to explain the more
bizarre design shortcomings in cars I've owned.  Like the Ford van that
had no coolant recovery bottle, just a hose pointing at the ground.

response 22 of 36:

Jul 11 03:58 UTC 2003

Re #18:  That used to be standard on everything.  So were vacuum motors
to run the windshield wipers.  Coolant bottles and electric motors are
the modern improvements (some mandated by law, some not).

response 24 of 36:

Jul 11 12:07 UTC 2003

Re #20:  No, Ford was neither supplier nor customer for that project.