Intoxication is not enough: the prosecution must also prove that the defendant was driving. This may be difficult if, as in the case of accidents, there are no witnesses to his being the driver of the vehicle.
Evidence will be suppressed if the officer did not have legal cause to (a) stop, (b) detain, and (c) arrest. Sobriety roadblocks present particularly complex issues.
Incriminating statements may be suppressed if warnings were not given at the appropriate time.
Implied consent warnings.
If the officer did not advise you of the consequences of refusing to take a chemical test, or gave it incorrectly, this may affect admissibility of the test results -- as well as the license suspension imposed by the motor vehicle department.
"Under the influence."
The officer's observations and opinions as to intoxication can be questioned -- the circumstances under which the field sobriety tests were given, for example, or the subjective (and predisposed) nature of what the officer considers as "failing". Too, witnesses can testify that you appeared to be sober.
There exists a wide range of potential problems with blood, breath or urine testing. "Non-specific" analysis, for example: most breath machines will register many chemical compounds found on the human breath as alcohol. And breath machines assume a 2100-to-1 ratio in converting alcohol in the breath into alcohol in the blood; in fact, this ratio varies widely from person to person (and within a person from one moment to another). Radio frequency interference can result in inaccurate readings. These and other defects in analysis can be brought out in cross-examination of the state's expert witness, and/or the defense can hire its own forensic chemist.
Testing during the absorptive phase.
The blood, breath or urine test will be unreliable if done while you are still actively absorbing alcohol (it takes 45 minutes to three hours to complete absorption; this can be delayed if food is present in the stomach). Thus, drinking "one for the road" can cause inaccurate test results.
This refers to the requirement that the BAC be "related back" in time from the test to the driving (see question #5). Again, a number of complex physiological problems are involved here.
Regulation of blood-alcohol testing.
The prosecution must prove that the blood, breath or urine test complied with state requirements as to calibration, maintenance, etc.