Whenever a motorist accused of driving while intoxicated agrees to take a breath test, the state’s star witness at trial will be the technical supervisor, the state expert on the breath test machine. One of the highlights of the state expert’s testimony is the calculation of his client’s alcohol concentration at the time of driving and the number of drinks he had to get there. This pseudoscientific theory is called retrograde extrapolation.

While the prosecutor asks the technical supervisor to determine these numbers for the jury, the technical supervisor will be sitting in the witness box furiously dialing numbers on a calculator. The whole testimony seems so dramatic, until he asks the state’s expert on cross-examination if he and the prosecutor discussed his testimony before taking the stand. When asked, you will be forced to admit that you calculated the numbers ahead of time.

Technical supervisor calculations can also be attacked by using the Texas Breath Alcohol Testing Program Operator’s Manual. Upon cross-examination, the state expert will testify that the manual has authority in the field of breath alcohol testing, and this will allow you to cross-examine the witness using the manual.

After the state expert has testified that your client’s alcohol concentration was higher at the time of driving than at the time of the test, you should ask the technical supervisor what your client ate or drank in the last hour before the traffic stop.

After the state expert admits that he cannot testify with any degree of certainty what his client consumed before being detained, ask him if he agrees with the manual’s statement that “there are three possibilities when estimating the concentration of alcohol in a moment before the test. “

When asked, the state expert will have to admit that the statement is true and that he cannot say with any degree of certainty whether his client’s alcohol concentration was higher, lower, or the same at the time of the test as it was at the time of testing. time to drive. .

Another area of ​​attack is the assumption that a person’s body eliminates alcohol at a steady rate. The state expert will testify that the average removal rate is .015 percent per hour. To calculate the driver’s alcohol concentration at the time of driving, the technical supervisor will simply multiply that number by the number of hours between the traffic stop and the breath test and add it to the breath test result. However, when crossing over, you will be forced to admit that there is no scientific evidence to support your claim that the clearance rate is constant. The rate of elimination is affected by the amount of food in the stomach at the time of drinking, the type of food in the stomach, stress levels, and physical ailments.

The key to questioning the state expert is not to make a frontal assault. The state pays the technical supervisor to testify on the accuracy and validity of the breath test and the breath test methodology. The wisest course of action is to attack the assumptions on which the state’s expert bases his testimony. Then, take the concessions the technical supervisor at the crossroads made and use them to create reasonable doubts during your closing argument, when it is too late for the state expert to defend your answers.

If you have been wrongly arrested for DWI, you need an experienced DWI attorney to defend your rights and restore your reputation. Contact Houston DWI attorney Paul B Kennedy for more information and a free consultation.