What happens during a roadside alcohol test

When a police officer suspects you have been drinking, they typically ask you to perform one or more tests at the roadside before deciding whether to arrest you. These tests fall into two categories: field sobriety tests, which measure balance and coordination, and breath tests, which measure the amount of alcohol in your system. The officer is looking for signs that your blood alcohol concentration (BAC) exceeds the legal limit — 0.08% in all U.S. states for drivers 21 and older.

You should know that roadside tests are voluntary in most states, though refusing can have consequences. The officer must have a reasonable suspicion that you are impaired before asking you to perform any test. This suspicion usually comes from observations like swerving, slurred speech, or the smell of alcohol.

Key Takeaways

  • Field sobriety tests measure balance and coordination and are voluntary in most states, though refusing may be noted in a police report.
  • Breath tests at the roadside use a handheld device and give a preliminary result, but are less accurate than tests done at a police station or hospital.
  • You have the right to refuse roadside tests in most states, but refusing a breath test after arrest may trigger automatic license suspension in your state.
  • If arrested, you will be offered a more formal test — either a breath test at the station or a blood test — and refusing that test carries serious legal consequences.
  • The legal BAC limit is 0.08% for drivers 21 and older, 0.04% for commercial drivers, and 0.02% for drivers under 21.

Field sobriety tests and what officers look for

Field sobriety tests are physical exercises designed to show whether alcohol has affected your balance, coordination, and ability to follow instructions. The three most common tests are the horizontal gaze nystagmus (HGN) test, the walk-and-turn test, and the one-leg stand test.

In the HGN test, the officer holds a pen or light in front of your eyes and moves it side to side while watching your eyes. Alcohol can cause involuntary eye movements, and the officer is looking for jerking motions. In the walk-and-turn test, you walk a straight line heel-to-toe for nine steps, turn, and walk back. The officer watches for stumbling, stepping off the line, or losing balance. In the one-leg stand test, you stand on one leg while the other is raised, and the officer times how long you can hold the position while watching for swaying or hopping.

These tests are not chemical measurements — they are observations of physical performance. They can be affected by fatigue, medical conditions, poor balance unrelated to alcohol, or even the surface you are standing on. You can decline to perform them, though the officer may note your refusal in their report.

Breath tests at the roadside versus at the station

If the officer believes you may be impaired, they may ask you to blow into a handheld breath-testing device at the roadside. This is called a preliminary alcohol screening (PAS) test or a roadside breath test. The device measures the amount of alcohol in your breath and estimates your BAC. The result is not admissible as evidence in court in most states — it is used only to help the officer decide whether to arrest you.

Roadside breath tests are less accurate than tests performed at a police station or hospital because the handheld devices are less precise and are not regularly calibrated the same way. If you are arrested, you will be taken to a police station or hospital and offered a more formal breath test or a blood test. This second test is the one that can be used as evidence in a DUI or DWI case.

In most states, you can refuse the roadside breath test without automatic legal consequences beyond what the officer notes in their report. However, refusing a breath test after arrest — the formal test at the station — is treated very differently and may result in automatic license suspension.

What happens if you refuse a test

Refusing a roadside field sobriety test or preliminary breath test is generally your right in most states. The officer cannot force you to perform these tests, and you can straightforward say "I do not consent to this test." The officer will likely note your refusal in their report, and it may be mentioned later, but refusal alone is not a crime.

Refusing a formal breath or blood test after arrest is a different matter. Many states have implied consent laws, which mean that by driving on public roads, you have already agreed to submit to a chemical test if arrested for suspected impaired driving. Refusing this test can trigger automatic license suspension — often for one year or longer — even if you are not convicted of a DUI or DWI. Some states also allow the refusal itself to be used as evidence against you in court.

A few states allow you to refuse a breath test but require a blood test instead, or vice versa. The specific rules vary by state, so understanding your state's law before you are in this situation is important.

How BAC limits work and who they explore to

Your blood alcohol concentration (BAC) is the percentage of alcohol in your bloodstream. It is measured as grams of alcohol per 100 milliliters of blood, expressed as a percentage. The legal limit for drivers 21 and older is 0.08% in all states. This means that if a test shows your BAC is 0.08% or higher, you can be arrested for driving under the influence.

However, you can be arrested for impaired driving even if your BAC is below 0.08% if the officer observes signs of impairment. Some states call this "driving under the influence" (DUI) and others call it "driving while intoxicated" (DWI), but the concept is the same.

Commercial drivers have a lower limit: 0.04%. Drivers under 21 have an even lower limit in most states: 0.02% or sometimes zero tolerance, meaning any detectable alcohol. These lower limits reflect the fact that alcohol affects younger and less-experienced drivers more severely, and that commercial drivers have a higher responsibility for public safety.

The difference between breath tests and blood tests

Both breath and blood tests measure BAC, but they work differently and have different levels of accuracy. A breath test estimates BAC by measuring the amount of alcohol in your breath. A blood test directly measures the amount of alcohol in your blood. Blood tests are generally considered more accurate because they measure the actual substance, while breath tests rely on an estimate based on the ratio of alcohol in breath to alcohol in blood.

Breath tests are faster and less invasive, so they are the standard test at police stations. Blood tests require a needle draw and are usually performed at a hospital or medical facility. In some states, if you refuse a breath test, the officer can request a blood test instead. In others, you have the right to request a blood test instead of a breath test.

Both tests can be challenged in court. Breath test results can be questioned if the device was not properly calibrated, if the officer did not follow the correct procedure, or if you burped or vomited shortly before the test. Blood test results can be questioned if the blood was not properly collected, stored, or tested, or if the lab made an error.

What to know about your rights during a traffic stop

You have the right to remain silent during a traffic stop, and you should exercise this right. Anything you say can be used as evidence against you. You can politely decline to answer questions beyond providing your license, registration, and proof of insurance. You do not have to explain where you are going, where you came from, or whether you have been drinking.

You have the right to refuse roadside field sobriety tests and the preliminary breath test. You do not have to perform these tests, and refusing them does not automatically mean you will be arrested — though the officer may arrest you based on other observations. If you are arrested and taken to a police station, you have the right to speak with a lawyer before submitting to a formal breath or blood test, though in practice this right is limited and varies by state.

If you are arrested, you will be read your rights (called Miranda rights), which include the right to remain silent and the right to a lawyer. Use these rights. Do not answer questions without a lawyer present, and do not sign anything without understanding what it says.

Frequently Asked Questions

Can I refuse a breath test at the roadside?

Yes, in most states you can refuse a preliminary breath test at the roadside. The officer may note your refusal, but it is not a crime. However, refusing a formal breath test after arrest at the police station may trigger automatic license suspension and other penalties under your state's implied consent law.

What if I have a medical condition that affects balance?

Medical conditions like inner ear problems, arthritis, or neurological disorders can affect your performance on field sobriety tests. If you have a condition that affects balance or coordination, you can tell the officer this before the test. You can also refuse the test and request a breath or blood test instead, which measures alcohol directly rather than physical performance.

How long does alcohol stay in your system?

The body metabolizes alcohol at a fairly consistent rate — roughly one standard drink per hour for most people. However, this varies based on body weight, food intake, metabolism, and other factors. A breath or blood test measures alcohol present at the moment of the test, not how much you drank earlier.

What is the difference between DUI and DWI?

DUI stands for "driving under the influence" and DWI stands for "driving while intoxicated." Different states use these terms differently. Some states use DUI for all impaired driving cases, while others use DWI for alcohol and DUI for drugs. Check your state's laws to understand which term applies where you live.

Can I be arrested for impaired driving if my BAC is below 0.08%?

Yes. You can be arrested if an officer observes signs of impairment — slurred speech, poor coordination, erratic driving — even if your BAC is below the legal limit. The legal limit is a threshold, not a safe level, and impairment can occur at lower BAC levels, especially for younger or less-experienced drivers.