Zero Tolerance Laws

Young drivers are less likely than adults to drive while impaired, but their crash risk is substantially higher when they do. Drivers are considered to be alcohol-impaired when their blood alcohol concentration (BAC) level is greater than 0.01 g/dL.

In 2020, 29% of young drivers (ages 15-20) involved in fatal crashes had blood alcohol concentrations (BACs) of .01 g/dL or higher; 82% of those young drivers who had alcohol in their systems had BACs of .08 g/dL or higher. A goal of public policy should be to enact zero tolerance laws with no acceptable BAC level. The aim of these zero tolerance laws is help prevent impaired driving.

While other countries have seen progress in alcohol-related crash deaths, the US as a whole has not. The US legal drinking limit is higher than in other countries, affecting the safety of all drivers, and among young drivers there is no safe limit for alcohol. It has long been known that any amount of alcohol is dangerous for young drivers and that the risk increases with greater amounts of alcohol in their system.

Alcohol affects reaction times and ability to drive safely as do other substances, like marijuana and prescription pills, that affect alertness. Youth who drive under the influence of alcohol or drugs put themselves and others at risk.  Adolescent drinkers are also more likely to ride as passengers of impaired drivers.

The dangers associated with alcohol and driving are exacerbated with the sadly common situation of binge drinking, a pattern of drinking that brings the person’s BAC to 0.8 grams percent or above in a short time – typically five drinks for men and four drinks for women in about two hours, according to the Centers for Disease Control and Prevention (CDC). This is a common dangerous practice, unfortunately, among adolescents who “pre-game” before going out.

Since 1988, all 50 states and the District of Columbia have implemented zero tolerance laws that set a limit of 0.02% BAC or lower for drivers under age 21. The 0.02 limit is equivalent to about one drink for the average person.

Although zero tolerance laws have been challenged based on their effectiveness, they have helped to significantly reduce the number of fatal crashes involving intoxicated young drivers. The first four states to reduce the legal BAC limit for young drivers (Maine, North Carolina, Wisconsin, and New Mexico) experienced a 34% decline in nighttime fatal crashes among young drivers. This decline was about one-third greater than a similar decline in four nearby states. An additional study of 12 states that passed zero tolerance laws reported a 20% reduction in single-vehicle nighttime fatal crashes among 15- to 25-year-old drivers.

Sixteen states have zero tolerance laws in effect for one or more drugs.

To learn more about zero tolerance laws in your state, visit the Mothers Against Drunk Driving website.

National Minimum Drinking Age Act

Until the 1980s, each state had its own laws determining the minimum drinking age. This led to the creation of “blood borders,” where teens would drive to states with lower drinking ages and be involved in fatal crashes on the return trip. In 1984, following an overall increase in alcohol-related traffic fatalities and injuries, Congress passed the National Minimum Drinking Age Act (NMDA), or the Federal Uniform Drinking Age Act, requiring states to legislate and enforce 21 as a minimum age for purchasing and publicly possessing alcoholic beverages. Since July 1988, all 50 states and the District of Columbia have enforced a legal drinking age of 21. The National Highway Traffic Safety Administration (NHTSA) estimates that Minimum Drinking Age laws have saved 31,959 lives since 1975.

Alcohol Policy Scale (APS)

To identify associations between alcohol policy environments and crash deaths among young people in the US, researchers used the Alcohol Policy Scale (APS) to rate alcohol policies and implementation in all 50 states and Washington DC. For each state and year, a panel of policy experts independently assessed each policy with regard to its efficacy for reducing excessive drinking or alcohol-related harm and the degree of legislative implementation for each policy.

States were ranked on how restrictive their alcohol policies were, including higher alcohol taxes and zero-tolerance policies for drivers younger than age 21. Then they cross-referenced the APS scores with crash data from the Fatality Analysis Reporting System (FARS). A 10 percentage-point increase in a state’s APS score was associated with a 9 percent decrease in the odds of alcohol-related crash fatalities. This research shows  the effect of comprehensive alcohol policies on alcohol-related MVC fatalities among young people.

Marijuana Driving Laws

Since 2012, 18 states and the District of Columbia have legalized marijuana for adults over the age of 21, and 37 states have legalized medical marijuana.  As more states pass legislation for legalization of marijuana either for recreational or medical use, understanding how THC (tetrahydrocannabinol), the active ingredient in marijuana, impacts driving should be a priority for the scientific community.

Although the research is limited, generally accepted results reveal a two-fold risk of a motor vehicle crash among drivers with any THC in the bloodstream. THC is fat soluble, and the ingredient can stay in a regular user's system several days after last using marijuana. THC metabolites, compounds created as the body processes THC, can remain in a person's system for weeks after last using marijuana.  

While laboratory studies of people with THC in their bloodstream do not support significant impairment on single tasks, such as memory, addition, or subtraction, there may be more significant impact on multitasking and handling unexpected events (which are critical components of safe driving). Teen drivers should be especially wary of driving after taking any substance that may be mind-altering, including marijuana. The tasks required for driving are complex and require much higher order thinking than they realize.

A 2022 study published in JAMA Psychiatry found that smoking cannabis can impair a person's ability to drive up to 4 hours after the drug is used. Researchers from the Center for Medicinal Cannabis Research at the University of California San Diego looked at 191 regular cannabis users and determined that smoking cannabis led to a significant decline in Composite Drive Scores (CDS) that assessed driving variables, including following a lead car at various speeds, swerving in their lane, and responding to divided attention tasks.

Detecting clinically significant levels of THC in the body can be complicated. Urine levels, which in many states are equated to blood levels (and are both illegal), can reflect past use and may not reliably detect people who are actually "high".

As far as marijuana driving laws, 17 states have zero tolerance or non-zero per se laws for marijuana. Zero tolerance laws prohibit driving with any measurable amount of marijuana in the body. Per se laws make it illegal to drive with amounts of marijuana in the body that exceed state limits:

  • 17 states have zero tolerance or non-zero per se laws for marijuana.
  • 10 states have zero tolerance for THC or a metabolite.
  • 3 states have zero tolerance for THC but no restriction on metabolites.
  • 4 states have specific per se limits for THC. For example, Georgia provides an exception to the per se standard for people who are “legally entitled to use” marijuana and other substances other than alcohol.
  • 1 state (Colorado) has a permissible inference law for THC. What this means is that the jury can infer that a person is guilty of driving under the influence (DUI) if the person has a set amount of THC in his or her blood. 

In 10 states, laws are in place related to driving with cannabis in motor vehicles. These laws are similar to "open container" statutes that limit alcohol consumption in motor vehicles. For more information on these state statutes, visit the National Conference of State Legislatures website.

To learn more about marijuana legalization laws, visit the Insurance Institute for Highway Safety website.  


In 2020, 29% of young drivers (ages 15-20) involved in fatal crashes had blood alcohol concentrations (BACs) of .01 g/dL or higher; 82% of those young drivers who had alcohol in their systems had BACs of .08 g/dL or higher.