Legal EvidenceThe District Attorney will likely have an array of evidence to use against you in a trial. Here is a list of the most common sources of evidence.
A breathalyzer (or breathalyser) is a device for estimating blood alcohol content (BAC) from a breath sample. "Breathalyzer" is the brand name of a series of models made by one manufacturer of these instruments, but has become a genericized trademark for all such instruments. Intoxilyzer, Intoximeter, AlcoScan, Alcotest, AlcoSensor, Alcolizer, Datamaster are the other most common brand names in use today. The U.S. Government's National Highway Traffic Safety Administration maintains a "Conforming Products List" of breath alcohol devices approved for evidentiary use, as well as for preliminary screening use.
These instruments however are not without their ability for error. Breath testers can be very sensitive to temperature, for example, and will give false readings if not adjusted or recalibrated to account for ambient or surrounding air temperatures. The temperature of the subject is also very important.
Also your breathing pattern can also significantly affect breath test results. One study found that the BAC readings of subjects decreased 11 to 14% after running up one flight of stairs and 22–25% after doing so twice. Another study found a 15% decrease in BAC readings after vigorous exercise or hyperventilation. Hyperventilation for 20 seconds has been shown to lower the reading by approximately 32%. On the other hand, holding your breath for 30 seconds can increase the breath test result by about 28%.
Lastly one of the most common sources of error in breath alcohol analysis is simply testing the subject too early—while his or her body is still absorbing the alcohol. Absorption of alcohol continues for anywhere from 45 minutes to two hours after drinking or even longer. Peak absorption normally occurs within an hour; this can range from as little as 15 minutes to as much as two-and-a-half hours.
Failure of law enforcement officers to use the devices properly or of administrators to have the machines properly maintained and re-calibrated as required are particularly common sources of error. However, most states have very strict guidelines regarding officer training and instrument maintenance and calibration.
Sobriety checkpoints or roadblocks involve law enforcement officials stopping every vehicle (or more typically, every nth vehicle) on a public roadway and investigating the possibility that the driver might be impaired to drive. They are often set up late at night or in the very early morning hours and on weekends, at which time the proportion of impaired drivers tends to be the highest.
With a portable and quick alcohol breath test, the police can test all drivers (if the law permits), and process the cars one by one as in a conveyor belt. When there is no quick test, a more complicated routine is necessary. Upon suspicion, the stopped driver is required to exit the vehicle and take a roadside sobriety test that requires the demonstration of both mental and balance skills. If the officer determines that the test has not been passed, the driver is then required to take an alcohol breath test (referred to as a Breathalyzer test in the United States).
The legality on this has many shades of grey. According to the 4th amendment to the United States Constitution, its states that: “The right of the people to be secure in their persons, houses, papers, and effects, against unreasonable searches and seizures, shall not be violated, and no Warrants shall issue, but upon probable cause, supported by Oath or affirmation, and particularly describing the place to be searched, and the persons or things to be seized.” Thus the Constitution would appear to prohibit people from being stopped without a search warrant or at least without probable cause that they have committed a crime.
The Michigan Supreme Court had found sobriety roadblocks to be a violation of the Fourth Amendment. However, by a 6-3 decision in Michigan Dept. of State Police v. Sitz (1990), the United States Supreme Court found properly conducted sobriety checkpoints to be constitutional. Although acknowledging that such checkpoints infringed on a constitutional right, Chief Justice Rehnquist argued that the state interest in reducing drunk driving outweighed this minor infringement.