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Phase 1: Cops and Traffic Stops

Each year, nearly a million people are arrested for impaired driving. Though laws and processes vary by state, each of them goes through a similar experience after an officer observes suspicious driving: 

  1. Traffic Stop 
  2. DUI investigation
  3. Arrest
  4. Chemical testing
  5. Drug Recognition Expert Evaluation
  6. Booking

After making a traffic stop, the officer documents his/her observations. During this preliminary DUI investigation, the officer must identify and document signs and symptoms of impairment to build a strong impaired driving case. This includes observing the driver’s behavior and performance on the Standardized Field Sobriety Tests (SFST), and any preliminary chemical screening results, such as preliminary breath tests or onsite oral fluid screening. 

Once the officer has completed the roadside investigation and believes that there is enough evidence of impairment to establish probable cause, an arrest is made, and the suspect is transported to the police station. 

At the station, the detained suspect is subjected to additional questioning and is required to provide an evidential breath test, which is one type of chemical testing. If he/she refuses to voluntarily submit to a breath test or a blood test, a warrant must be obtained to authorize a non-voluntary blood draw. If a blood draw is required, the suspect will be transported to a healthcare facility to have the blood drawn by a medical professional. In some jurisdictions, law enforcement may be trained in phlebotomy and the blood draw can be performed at the station.   

If the person provides a breath sample that tests below the legal limit or the officer suspects that the subject’s impairment is caused by drugs as opposed to or in addition to alcohol, a drug recognition expert (DRE) will likely be called to perform a 12-step standardized drug evaluation to determine what category or categories of drugs are causing the observed impairment. In drug cases, a urine test or blood draw is required. Once the investigation is complete, the DUI suspect is booked into the detention facility, typically a county jail. 

Traffic Stop

To conduct a traffic stop, an officer must have reasonable suspicion that a traffic violation or criminal activity has occurred or is occurring. Signs of impaired driving include:

  • Problems maintaining proper lane position—weaving, swerving, drifting, crossing center and lane lines, varying speed, acceleration
  • Vigilance issues—wrong way driving, failure to obey road signs or traffic lights, driving at night without headlights
  • Judgment issues—following too closely, improper turns

DUI Investigation

Once the motorist has been stopped, the officer can observe the driver up close and ask questions. The officer must decide if there is sufficient cause to instruct the driver to step out of the vehicle. Indicators of impairment include:

  • Difficulty with motor controls
  • Fumbling with license/registration
  • Repeating questions or comments
  • Slurred speech,
  • Slow to respond to commands 
  • Changing answers to questions
  • Odor of alcohol
  • Evidence of alcohol consumption or drug use in the vehicle

Once the officer gives the command to exit the vehicle, he or she will observe the driver’s behavior and document actions. Does the individual have difficulty exiting? Lean on objects? Balance issues?

If the officer continues to believe the subject is impaired, the next step is to administer the Standardized Field Sobriety Tests (SFSTs).

Standardized Field Sobriety Tests


Standardized Field Sobriety Tests were developed and validated through research funded by the National Highway Traffic Safety Administration (NHTSA) and have withstood multiple court challenges. They are a three-test battery comprised of horizontal gaze nystagmus (rapid eye movements), the walk and turn, and the one leg stand. Officers are trained to administer these tests in a standardized and systematic manner. The tests contained in the SFST battery are designed to identify impairment, not the source of the impairment.


In some jurisdictions, an officer may also ask a driver to provide a preliminary breath test (PBT) as part of the initial investigation. In most jurisdictions, the test is voluntary, and an individual can refuse to provide a breath sample. Oral fluid technology can also be used to screen for drugs at roadside, although this practice is not commonplace.


To make an arrest, probable cause must be established during the investigation. Probable cause is based on the “totality of the circumstances.” This means that the officer has collected enough evidence through observations, the individual’s performance on the SFSTs, and preliminary breath test results to support the belief that the driver is intoxicated. 

The arrested individual is transported to a police station or sheriff’s office for evidential chemical testing. In cases where the arresting officer suspects that the individual is impaired by drugs, a DRE may be called to meet them at the station to perform a drug evaluation.

In most cases, the DUI investigation will end once the officer is able to build a strong alcohol-impaired driving case. A DRE is likely to be called only in cases where an officer sees symptoms of impairment that are inconsistent with an alcohol case or at low BAC levels.

Chemical Testing

Several testing methods are used in DUI cases. These include evidential breath, blood, or urine tests.. The Supreme Court has ruled that officers could compel breath tests without a warrant because breath testing is non-invasive. If a person refuses to submit to a breath test or if the DUI investigation involves drugs, the other evidential options available are blood and urine testing. Both specimen collections are intrusive, require officers to handle biological samples, and are relatively expensive. 

Blood is a much more common testing method and is considered to be the “gold standard” as the results are conclusive, sensitive, and specific. Moreover, alcohol levels in blood strongly correlate with levels of impairment. In drug cases, positive results are indicative of recent use. 

Unfortunately, officers often encounter issues when collecting blood samples. Only medically trained professionals can collect blood samples, which means that if officers are not trained and certified as phlebotomists, suspects must be transported to a medical facility. 

Due to recent Supreme Court rulings, warrants are generally required for blood draws in DUI cases. The Court has noted that advancements in technology make it far easier for law enforcement officers to obtain warrants quickly and that the natural dissipation of alcohol levels in one’s blood did not create an exception to the Fourth Amendment’s warrant requirement based on exigent circumstances.  

The Supreme Court has held that officers may compel blood samples from DUI suspects only if (1) they have probable cause to believe the driver operated his or her vehicle while impaired; and (2) they obtain

a warrant to seize one’s blood or the driver voluntarily consents to a blood draw or one of the traditional exceptions to the warrant requirement exists. 

Perhaps the greatest concern expressed by law enforcement officers is the amount of time that passes between when the traffic stop is made and when the blood sample is finally collected. In many jurisdictions, between up to two hours can pass before the officer is able to make the arrest, obtain a warrant, and transport the suspect to a facility to have the blood draw performed. During this time, alcohol and/or drugs are dissipating from the driver’s body. The blood alcohol or drug concentration levels measured in the laboratory test are therefore lower than at the time of the impaired driving incident. 

The advent of electronic warrant systems can address this last issue. These systems can significantly streamline the arrest process and reduce the amount of time between the request, approval, and execution of the warrant. The greatest advantage of these systems is that they provide a mechanism for officers to obtain accurate BAC or toxicology results quickly thereby preserving the chemical evidence. To learn more about these systems, see “Critical DUI System Reforms: Law Enforcement.”

Urine testing is no longer a preferred method. Urine results do not correlate as well with impairment as blood because its window of detection can extend for days, especially in the case of drugs like cannabis. Also, only a gender-appropriate officer can collect a sample and it can take suspects several hours to provide a specimen. Just a handful of states continue to rely on urinalysis in DUI cases. 

Drug Recogntion Expert Evaluation

In cases where law enforcement officers detect signs of significant impairment but the subject’s BAC is well under the legal limit, a drug recognition expert (DRE) is often called to perform a drug evaluation.

The Drug Evaluation and Classification (DEC) Program was established in the 1970s by the Los Angeles Police Department. The LAPD collaborated with medical professionals to develop a procedure aimed at identifying drug impairment. These efforts led to the creation of a 12-step standardized protocol and DREs. The National Highway Traffic Safety Administration (NHTSA) and the International Association of Chiefs of Police (IACP) oversee the program and its training curriculum. For an officer to become a DRE, he or she must participate in 100+ hours of intensive classroom instruction, formal training, and field certification. Once certified, a DRE must continue to engage in training and be re-certified every two years. 

Every state currently has a DRE program and in recent years, many states have increased the number of officers that have earned this rigorous certification. Together with the Governors Highway Safety Association, has provided funding to train more than 2500 law enforcement officers in 16 states to detect drug-impaired and polysubstance-impaired driving. The NHTSA and the IACP have recently embarked on a similar multi-million-dollar training effort. Unfortunately, DREs are not always available to perform evaluations, particularly in rural jurisdictions or small departments. 

The first goal of any drug evaluation is to determine if a suspect is impaired. The officer will then try to  determine if the impairment can be attributed to a medical condition or drug use. The DRE will then render an opinion about what category of drug or combination of categories is the likely cause of the impairment There are seven categories of drugs that DREs consider: central nervous system (CNS) depressants, CNS stimulants, hallucinogens, dissociative anesthetics, narcotic analgesics, inhalants, and cannabis. Each DRE administers a standardized 12-step process:

  1. Breath alcohol test
  2. Interview of the arresting officer
  3. Preliminary examination and first pulse check
  4. Eye examination
  5. Divided attention psychophysical tests
  6. Vital signs and second pulse check
  7. Dark room examinations
  8. Examination for muscle tone
  9. Check for injection sites and third pulse check
  10. Subject’s statements and other observations
  11. Analysis and opinions of the evaluator
  12. Toxicological examination

DRE testimony has been accepted as expert testimony and withstood most evidence admissibility challenges.

DUI Booking

The final step is the booking of the suspect. The individual is fingerprinted, has a mugshot taken, and is placed in a holding cell until their first appearance in court. Typically, a suspect is arraigned within 24 hours of being detained.