The following is a list of symptoms in descending order of probability that the person observed is driving while intoxicated. The list is based upon research conducted by the National Highway Traffic Administration:

• Turning with a wide radius
• Straddling center of lane marker
• "Appearing to be drunk"
• Almost striking object or vehicle
• Weaving
• Driving on other than designated highway
• Swerving
• Speed more than 10 mph below limit
• Stopping without cause in traffic lane
• Following too closely
• Drifting
• Tires on center or lane marker
• Braking erratically
• Driving into opposing or crossing traffic
• Signalling inconsistent with driving actions
• Slow response to traffic signals
• Stopping inappropriately (other than in lane)
• Turning abruptly or illegally
• Accelerating or decelerating rapidly
• Headlights off

Speeding, incidentally, is not a symptom of DUI; because of quicker judgment and reflexes, it may indicate sobriety

The traditional symptoms of intoxication taught at the police academies are:

• Flushed face
• Red, watery, glassy and/or bloodshot eyes
• Odor of alcohol on breath
• Slurred speech
• Fumbling with wallet trying to get license
• Failure to comprehend the officer's questions
• Staggering when exiting vehicle
• Swaying/instability on feet
• Leaning on car for support
• Combative, argumentative, jovial or other "inappropriate" attitude
• Soiled, rumpled, disorderly clothing
• Stumbling while walking
• Disorientation as to time and place
• Inability to follow directions

Again, this varies according to the laws of the state and the customs of the local jurisdiction. Generally speaking, a conviction for a first offense may involve a fine, a license suspension or restriction, attendance at a DUI education course for a period of time, and probation for perhaps three years. A short jail sentence may or may not be required; for a second offense, it almost certainly will. Additional punishment may involve community service, ignition interlock devices, AA meetings, and/or impounding of the vehicle.

The traditional offense is "driving under the influence of alcohol" (DUI), or in some states, "operating while intoxicated" (OWI), or "driving while intoxicated" (DWI). In recent years, however, 49 states have also enacted a second, so-called "per se" offense: driving with an excessive blood-alcohol concentration (either .08% or .10%). In those states, BOTH offenses are charged. The defendant can even be convicted of both, but can be punished for only one.

If the case involves a refusal to submit to chemical testing, of course, only the traditional offense will be charged.

Potential defenses in a given drunk driving case are almost limitless due to the complexities of the offense. Roughly speaking, however, the majority can be broken down into the following areas:

• Driving. Intoxication is not enough: the prosecution must also prove that the defendant was driving. This may be difficult if, as in the case of some accidents, there are no witnesses to his being the driver of the vehicle.
• Probable cause. 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.
• Miranda. 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, in some states this may invalidate a DMV license suspension based upon a refusal to provide a breath/blood sample.
• "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.
• Blood-alcohol concentration. 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 30 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.
• Retrograde extrapolation. This refers to the requirement that the BAC be "related back" in time from the test to the driving. 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.
• License suspension hearings. A number of issues can be raised in the context of an administrative hearing before the state's department of motor vehicles.