A Forensic take on Coffey v. Shiomoto

A Forensic Take on Coffey v. Shiomoto

On April 6th, 2015, the California Supreme Court decided that the subjective observations of a police officer regarding driving and field sobriety tests can carry more weight than the expert opinion of a forensic toxicologist and the reasonable inferences to be made from objective chemical tests when deciding whether a person’s BAC is over the legal limit at a DMV Administrative Per Se hearing.

The Case:

The case concerned a DUI arrest and the subsequent Department of Motor Vehicles hearing to contest the administrative driver’s license suspension. The defense called a forensic toxicologist to interpret the chemical test results, which consisted of two breath tests given at 56 and 59 minutes after driving, with results of 0.08 and 0.09%, followed by a blood test an hour and 23 minutes after driving which had results of 0.095% and 0.096%. The forensic toxicologist interpreted these results to mean that the subject’s blood alcohol level was rising over that time period and that the subject’s BAC at time of driving was more than likely under the 0.08% threshold.

In typical fashion, the DMV hearing officer, in the written opinion, said the expert’s conclusions “were not supported by reliable evidence, were ‘too speculative to support the contention,’ and were ‘based on a subjective interpretation of the evidence.” (Slip Opinion page 6). The hearing officer further criticized the expert for not examining the breath testing device used in the case, offering no opinion regarding whether it was working correctly, and did not show that any other experts in the scientific community had reached similar conclusions. (Slip Opinion at Page 7). The hearing officer then based her decision on the observations of the officer regarding the subject’s driving and field sobriety test performance to infer that the circumstantial evidence was more supportive of a conclusion that the subject was over 0.08% at the time of driving.

The California Supreme Court sided with the hearing officer.

What’s wrong with the decision from a scientific point of view?

The court correctly noted (on pages 17 and 18) that a chemical test at a later time does not tell you what the BAC was at the time of driving. It might be higher or lower. Circumstantial evidence will be required to help determine which. The question the court got wrong is what circumstantial evidence is stronger. The chemical test taken after arrest is circumstantial evidence of the BAC at the time of driving. So is the PAS test taken before arrest. The Court ruled that driving, FST’s and behavior are also relevant evidence of the BAC at time of driving. While this is true, the court erred in agreeing with the Hearing Officer that these observations may be given more weight than TWO sets of chemical tests.

The issue here is the defense that the subject’s BAC may have been lower than the legal limit at the time of driving, but over at the time of the chemical tests (a defense that the court disfavors, citing to the legislative history of CVC 23152(b) which calls it a loophole)(Slip Opinion at 11). There are two scenarios which come before the DMV at these administrative hearings: those where the subject took only the post arrest chemical test, either blood or breath, and those where the subject took both a pre-arrest breath test (a PAS test) and a post arrest chemical test. In the first scenario, in order to establish if a person’s BAC was rising or falling at the time of driving, the only circumstantial evidence of BAC at the time of driving besides the post arrest chemical test is the driving, FST’s, behavior, and the subject’s statements about drinking. When offered in these cases, the DMV routinely disregards these factors are speculative, uncertain, and insufficient to rebut the presumption that the BAC was higher than 0.08 if the test was taken within three hours (a presumption the court would not decide if applied at DMV hearings). The other scenario is when there are two sets of chemical tests. In some of these cases, the change in value of the chemical tests clearly indicates that the subject’s BAC is rising over the time interval between the tests, and therefore was certainly lower at the earlier time of driving. Faced with this evidence, the DMV then wants to rely on the other circumstantial evidence that it routinely dismisses in the single chemical test cases to dismiss the clear inference of lower BAC in the two chemical test cases. In this matter the California Supreme Court says that’s just fine, but that is where it went wrong.

The real question here was how the hearing officer weighed the circumstantial evidence, not whether she could weigh it in the first place. Given the choice between two sets of chemical test results on the one hand and driving, FST’s, behavior, and statements on the other, scientifically speaking, there is no contest – properly conducted chemical tests are much more reliable.

Before we discuss the weighing of the evidence, we should, as Socrates required, define our terms, especially terms that have different meanings but are sometimes used interchangeably when it suits somebody’s purpose.

Impairment: A decrease in performance or ability, however slight.

Under the Influence: Sufficient impairment to warrant legal sanction.

Intoxicated: Significantly affected by an impairing substance.

Under the influence is a legal term of art. It has a legal definition in California: when the subject is so affected by alcohol (or other drug) as to be unable to drive with the care and caution of a sober person under the same or similar circumstances. This rather tortuous definition allows that there is some amount of impairment that occurs before the subject reaches 0.08%, but that isn’t sufficient impairment to warrant legal sanction since the legislature has prohibited driving at a BAC over 0.08%, it is the amount of impairment that occurs at 0.08% that impairs driving to the requisite degree. The Court called 0.08 “the threshold at which a person cannot safely operate a motor vehicle due to alcohol consumption.” (Slip Opinion at 21). In his concurrance, Justice Liu notes that “the fact that 0.08 percent BAC is a threshold associated with an unsafe degree of impairment does not imply tht no impairment occurs below that threshold.” (Slip Opinion Concurrance at page 1). In fact, Justice Liu’s concurrance makes a very interesting point: in his opinion, a lack of impairment tends to show that the BAC was under 0.08%, but evidence of impairment (unless extreme) does not show that the BAC was over 0.08% because impairment can occur below that level, and there is no direct correlation between impairment and BAC. (Slip Opinion Concurrance at page 1). What the majority gives, the concurrnace tries to take away, or at least severly limit.

So it is vitally important to understand that a person may exhibit some impairment that does not rise to the level of being “Under the Influence.” The court is very clear that a person is not “under the infuence” until the level of impairment is comparable to that which typically occurs at 0.08%. There are levels of impairment that are not legally unsafe.

The weighing:

 Here is where, in my opinion, the decision doesn’t make sense. Lets look at the evidence the Department had to weigh. On the one hand, there are four chemical test values, and while they are all over the legal limit, they are also rising from pre-arrest to post arrest. Given the extensive literature on the BAC-time curve, it is obvious that the only way this can occur is if at least the PAS the tests were taken on the ascending limb of the curve. The Attorney General would like to dismiss the differences as an effect of margin of error, and the difference between blood and breath tests, but DMV doesn’t give the defendant the benefit of margin of error, so it shouldn’t be allowed to argue that to its own benefit, and California consideres blood and breath tests to be equivilent (see People v. VanGelder). These tests are objective. They give a number. They are not subjective, they do not depend on a person’s perception or opinion about them. They stand on their own.

On the other hand, there is the driving, FST’s, and behavior. All of these are subjective. They depend on the officer’s opinion, and his perception of the events. They are subject to interpretation, and are non-specific, in that there are many reasons why a person may show various symptoms and behaviors besides alcohol impairment. Secondly, they are only very indirect indicators of BAC, as noted by Justice Liu. Their absence indicates a low or no BAC, because there isn’t impairment, but their presence does not differentiate in close cases. You can’t use FST’s to tell a 0.07 from a 0.08, or a 0.08 from a 0.09.

So the court is saying that the DMV can weigh the circumstantial evidence in direct opposition to its actual value. The chemical tests are direct indicators of BAC at the time they are taken, and in most cases much better indicators of the BAC at other times, given the well settled science of the general shape of BAC time curves, than the other less reliable evidence of driving, FST’s and behavior. Yet the DMV can use those less reliable indicators to rebut the more reliable ones.

 Where do we go from here?

 This opinion makes expert testimony in rising cases all the more critical, and limits when such defenses are going to be viable. However, it does specify factors that need to be addressed by toxicologists testifying in these matters.

First, the toxicologist must examine the calibration and maintenance records for all chemical test devices used, and the procedures followed, to be able to make an opinion as to the accuracy and reliability of the results, especially of the PAS results, which are not granted the presumption of reliability that the post arrest chemical tests receive.

Second, the toxicologist must evaluate the circumstantial evidence of BAC, including the driving, FST’s and behavior of the subject, as well as the subject’s statements. Here, it will be important to provide scientific support for any conclusions reached to show that other scientists in the field would come to the same conclusion. It will be especially important to give literature citations to support the toxicologist’s conclusion, and be prepared to explain why the chemical test evidence is so much more reliable and compelling than the other, weaker circumstantial evidence on which the Department might prefer to rest its case.

Third, it is important to define subjective and objective views of the evidence, and distinguish between the subjective and objective measures that are available for interpretation. It will also be important to distinguish between speculation and estimation, and between speculation and uncertainty. The boilerplate language used in DMV opinions to discount expert testimony has routinely called it speculation and a subjective view of the evidence. The toxicologist must be prepared to explain why the opinions offered are neither subjective nor speculative.

Finally, it may be effective to have the toxicologist prepare a scientific report, complete with citations to peer review literature, to support the opinions offered, in the same way an attorney might offer a brief in suport of closing argument. In addition, there may be times where doing a study of a subject under conditions similar to the night of the offense to show the duration of alcohol absorption in that subject may have value, since the DMV hearing officer criticized the expert for not having done any studies or experiments.


While the Court’s decision gives the DMV free reign to weigh the circumstantial evidence as it sees fit, even when contrary to the clear import of the more scientific evidence, the Court does provide some guidelines for how to approach such cases in the future to make at least a more compelling record on review.


You are a criminal defense attorney. You have a DUI case. You think there may be issues in the lab work, but you’re just not sure how to find them. You are at the DMV hearing, and you need to rebut the presumption of the BAC at the time of driving. You are in trial, and you need to get your side of the story in front of the fact finder, and the Government’s expert is just not cooperating.

Impaired Driving Toxicology can help. I was a forensic scientist for over 18 years, a reserve deputy sheriff for 14 years, and a DUI defense attorney for 7 years. I have the education, training, and experience to make a difference in your cases. I am offering case review and expert testimony in DUI cases, presently involving either alcohol or marijuana, and case review and consultation only in matters involving other drugs and Drug Recognition Expert examinations.

I base my opinions primarily on sound peer-reviewed scientific journal articles, supported by my own experience. Because of that, there are some defenses that I can’t support. Also, I am not prepared to critique specific breath testing instruments or their source code. There are other experts who can do that. I am, however, very familiar with the physiological aspects of breath testing and the issues that affect breath testing instruments in general. I am also very familiar with blood testing by headspace gas chromatography, field sobriety testing, and alcohol and marijuana metabolism and impairment.

It has been written: “we see the world not as it is, but as we are.” I take a broader perspective on the science than the government’s experts because I have had the experience of seeing the science from both sides and for a long time. I went to law school, not to practice law, but to become an educator on forensic science. It seemed that there was a gap in understanding between the scientists and the lawyers. We were speaking different languages. I hoped that if I learned to speak lawyer, I could bridge that gap. That is what I’m going to do now. Come back often. I’ll be posting about issues in DUI toxicology to help attorneys understand DUI science, its applications and limitations.

Ronald L. Moore,

Impaired Driving Toxicology