Holy junk science, BAT van! Houston's portable breathalyzers accuracy questioned

From typical discussions of forensic errors in DNA exonerations, etc., you might think they occur mainly in the most serious crimes like rape, murder, etc., but law enforcement's use of flawed science may extend to more workaday crimes like DWI as well. Reported the Houston Chronicle recently ("Controversy continues to dog BAT vans," Sept. 8):

A driving while intoxicated case that sparked doubts on the accuracy of test results from the Houston Police Department's breath alcohol testing vans has been dismissed, while evidence from the vans in at least two other cases has also come into question.

A former HPD crime lab supervisor testified during a court hearing in July that she quit because she could not trust the accuracy and integrity of breath alcohol tests from the department's breath testing vehicles. Since then at least two other defense attorneys ...  say evidence in DWI cases they are handling could have been compromised because of the problems with the vehicles.

During the testimony in July, the former HPD lab supervisor, Amanda Culbertson, said the breath alcohol testing vehicles, also known as BAT vans, incurred such electrical problems as overheating. Those problems affect gauges, she said, and can alter the control sample used to calibrate the breath-test machine in the vans, possibly affecting the accuracy of test results.

HPD officials have acknowledged there have been problems, including air conditioning, in the BAT vans since they were purchased in 2008, but said that no cases should be compromised as a result of the temperature in the BAT vans.

But defense attorney Mark Thiessen said he does not believe that the breath testing machines were working properly in the BAT vans and that the tests were not run under proper protocols.

Paul Kennedy had an excellent post in August explaining how changes in temperature can affect such tests if the comparison sample isn't just right. Notably, the company that makes the breathalyzer won't actually reveal the source code behind its analysis, nor guarantee beyond one year that their products will be free from defects in material and workmanship. Even when working properly, the margin of error for some versions of the instrument is up to 25%.

Concerns about the accuracy of breathalyzer tests have been raised for several years now in Houston and elsewhere, but the practical implications of how many cases would be affected if they were deemed untrustworthy have scared away elected judges from closely interrogating the technology. Between the political clout of groups like MADD and the fact that so much government employment, fine income, and even trauma hospital funding ride on a steady stream of DWI revenue, judges are no more likely to question breath-test results than officials in Salem would have questioned that dunking in water might expose witches.

See also: Paycheck vs. Integrity: Houston PD crime lab supervisor resigns over faulty breathalyzers, feared retaliation