Motorists accused of drunk driving can now present evidence that the ratio used by the machine's computer program to calculate the amount of alcohol in their blood that the machine doesn't take into account the driver's own partition ratio, temperature, sex or medical condition. They may also challenge the precision of the machine.
In People v. McNeal, S157565 (July 9, 2009), the Court held that a person charged under the state's "generic" DUI statute (loss of normal use...) has the right to challenge the accuary of the breat test machine, but that a person charged with a per se violation does not because the state mandated the 2100:1 partition ratio in the statute. Mr. McNeal was charged with violating both the generic statute and the per se statute. When he attempted to blow into the machine, it took five blows to produce two valid samples, both reading .10.
The jury convicted on the generic charge but hung on the per se charge.
"Evidence casting doubt on the accuracy of the breath-to-blood conversion ratio is just as relevant as other evidence rebutting the presumption of intoxication from a breath test result, such as evidence that the defendant had a high tolerance for alcohol or performed well in field sobriety testing." Justice Carol A. Corrigan, California Supreme Court.
While we in Texas take for granted our right to challenge the assumptions behind the state's breath test machine, there are plenty of folks accused of DWI that don't have the right to do so. In California the jury is instructed that if a breath test taken within 3 hours of the traffic stop indicates a blood alcohol concentration of .08 or higher, then the jury may infer that the driver was over the legal limit at the time of driving.
I find it very troubling that in some jurisdictions around this country, juries are instructed that they must accept the results of a chemical test in a drunk driving trial. In the end, the state's breath test machine is just another witness; albeit one we don't have the ability to cross examine.
- California Supreme Court says DUI breath test accuracy can be weighed (San Jose Mercury-News, July 9, 2009)