The single most important factor in whether an individual will be arrested for driving under the influence (DUI) is not the evidence. It is the individual human differences of the officer himself.
A study by the National Highway Traffic and Safety Administration [U.S. Department of Transportation Report No. H5-801-230] points out the effect of these differences on an officers' observations and conduct in the field:
The officer's age and experience play a role in his alcohol-related arrest decisions. Younger officers, and those with relatively few years of seniority, tend to have a more positive attitude toward alcohol-related enforcement and make more arrests on that charge than do older officers. This result was found to hold true regardless of the type of department in which the officer serves or the specific type of duty to which he is assigned.
The officer's personal use of alcohol is inversely related to his level of alcohol-related enforcement. Patrolmen who drink make significantly fewer arrests than those who do not, and those who drink frequently make significantly fewer arrests than those who use alcohol only occasionally.
Lack of knowledge concerning the relationship between alcohol and intoxication is widespread among police officers and imparts a negative influence on alcohol-related enforcement. Most officers underestimate — often by a wide margin — the amount of alcohol a suspect would have to consume in order to achieve the statutory limit of blood-alcohol concentration.
Specialized training has a strong positive influence on alcohol-related arrests. Patrolmen who have received instruction in the operation of breath testing devices and/or in alcohol-related enforcement (particularly in municipal departments) were found to lack this specialized training.
Specialization in duty assignment can also enhance alcohol-related enforcement. Patrolmen assigned to traffic divisions, in particular, produce higher arrest rates than those charged with general patrol duties.
Near the end of the duty shift, alcohol-related investigations decrease substantially. This is particularly true in departments that have adopted relatively time-consuming procedures for processing alcohol-related arrests.
Weather conditions also affect alcohol-related arrests. There is encouraging evidence that foul weather has a positive influence on the attitude of many officers; they are more appreciative of the risk posed by an alcohol-related suspect when driving conditions are hazardous, and are less likely to avoid the arrest when those conditions prevail.
The suspect's attitude can have a strong influence on the arrest/no arrest decision. If the suspect proves uncooperative or argumentative, a positive influence for arrest results. Conversely, the likelihood of arrest decreases when the suspect seems cooperative.
The suspect's race is a key distinguishing characteristic in alcohol-related cases. The officers surveyed (the overwhelming majority of whom were white) reported releasing significantly more nonwhite suspects than they arrested. The data do not suggest that this reflects a greater tendency to exercise discretion when dealing with nonwhite drivers. Rather, the officers seem more willing to initiate an investigation when the suspect is not of their own race.
A suspect's age is another distinguishing characteristic of these cases, and patrolmen reported releasing significantly more young suspects than they arrested. This appears to stem from two distinct causes. First, young officers exhibit more sympathy for young suspects, i.e., seem less disposed to arrest a driver of their own age group. Second, older officers seem more willing to stop young suspects, i.e., are more likely to conduct an investigation when the driver is young, even if the evidence of alcohol-related violation is not clear.
The suspect's sex also plays a role in the arrest/no arrest decision. Patrolmen seem more reluctant to arrest a woman for alcohol-related violations, largely because processing of a female arrestee is generally more complex and time consuming.
Most DUI cases depend largely upon two variables: the officer and the machine. As has been discussed repeatedly in past posts, the machine is an unknown and unreliable variable. As this federal study indicates, so is the officer.