Distraction and impairment

Distraction and impairment both reduce a driver's ability to perceive hazards and respond in time. Distraction takes attention away from driving for a moment; impairment from alcohol, cannabis, other drugs or some medications degrades the brain's ability to drive for hours. Both are common, both are deadly, and both are among the leading contributors to serious collisions. This lesson explains how each affects driving, what the law says, and what drivers and employers can do. Fatigue, which impairs drivers in similar ways, is covered in lesson 6.

Three kinds of distraction

Distraction is anything that takes a driver's attention away from driving. It is often grouped into three kinds. Visual distraction takes your eyes off the road. Manual distraction takes your hands off the wheel. Cognitive distraction takes your mind off driving. Texting is so dangerous because it combines all three. But cognitive distraction alone is enough to cause collisions: drivers deep in a phone conversation, even hands-free, have been shown in many studies to miss more hazards and react more slowly than undistracted drivers, because the brain cannot fully attend to both tasks.

# Distance travelled "blind" while looking away (illustrative glance times).
for kmh in (50, 80, 100):
    v = kmh / 3.6
    for glance in (2, 5):
        d = v * glance
        print(f"{kmh:>3} km/h, {glance} s glance: {d:5.0f} m ({d * 3.281:5.0f} ft) travelled without looking")

 50 km/h, 2 s glance:    28 m (   91 ft) traveled without looking
 50 km/h, 5 s glance:    69 m (  228 ft) traveled without looking
 80 km/h, 2 s glance:    44 m (  146 ft) traveled without looking
 80 km/h, 5 s glance:   111 m (  365 ft) traveled without looking
100 km/h, 2 s glance:    56 m (  182 ft) traveled without looking
100 km/h, 5 s glance:   139 m (  456 ft) traveled without looking

Reading a short text message commonly takes around five seconds. At highway speed, that is more than the length of a football field traveled with eyes off the road. Research from naturalistic driving studies, in which cars were fitted with cameras and sensors for months, found that glances away from the road longer than about two seconds sharply increased crash and near-crash risk.

In-vehicle technology

Modern vehicles are full of screens: navigation, infotainment, fleet dispatch tablets and electronic logging devices. Many of these are useful, but every interaction takes attention. Good fleet practice is to set up navigation before departing, use voice prompts rather than looking at the screen, lock out or restrict dispatch tablets while the vehicle moves, and pull over safely to read job details or respond to messages. Advanced driver assistance systems, such as lane-keeping and adaptive cruise control, can reduce workload but can also tempt drivers to pay less attention. They are assistance, not automation, and the driver remains fully responsible.

The law on handheld devices

Nearly every state bans texting while driving, and a large and growing number ban all handheld phone use while driving, allowing only hands-free use. Many states ban all phone use by novice drivers and school bus drivers. For commercial motor vehicle drivers, federal FMCSA rules (49 CFR 392.80 and 392.82) prohibit texting and using a handheld mobile phone while driving, with civil penalties for drivers and for motor carriers that allow or require it, and repeated violations can disqualify a driver from operating a commercial vehicle. Check your state's rules, because they differ in detail.

Legal rules are a floor, not a target. Many employers adopt stricter fleet policies that forbid all phone use while driving, including hands-free, except for emergencies, and require drivers to pull over in a safe place before making or taking calls. They also tell supervisors and clients not to expect immediate replies from people who are driving.

Alcohol

Alcohol slows reaction time, impairs judgment, narrows attention, reduces coordination and makes drivers more willing to take risks. Impairment begins well below the legal limits. Even a blood alcohol concentration (BAC) of 0.02 to 0.05 percent measurably affects divided attention and tracking, and crash risk rises steeply above 0.05 percent. Only time removes alcohol from the body; coffee, cold showers and food do not speed it up. On average the body eliminates roughly one standard drink an hour, but individuals vary widely.

Every state sets a per se limit of 0.08 percent BAC for adult drivers of ordinary vehicles, except Utah, which lowered its limit to 0.05 percent in 2018. Drivers of commercial motor vehicles are subject to a federal limit of 0.04 percent, and a CMV driver with any detectable alcohol (0.02 or above on a test) must be removed from duty for 24 hours. Drivers under 21 face zero-tolerance limits. Refusing a test leads to automatic license suspension under implied consent laws.

Cannabis and other drugs

Cannabis impairs coordination, reaction time, attention and the ability to judge time and distance. Its effects vary with the product, the dose, how it is consumed and the person. Edible products can take an hour or more to take effect and last many hours, which makes it hard for users to judge when they are fit to drive. Combining cannabis and alcohol is particularly dangerous because the effects add up. Other illegal drugs, such as opioids, stimulants and hallucinogens, impair driving in different ways, and some, such as methamphetamine, can make drivers aggressive and overconfident, followed by severe fatigue as the drug wears off.

Drug-impaired driving is illegal in every state. Some states set per se limits for THC or other drugs, while others require proof of impairment or have zero-tolerance rules. Because THC levels in blood do not track impairment as reliably as alcohol does, enforcement often relies on drug recognition experts. For commercial drivers, federal rules prohibit the use of marijuana and other Schedule I drugs regardless of state legalization, and DOT drug tests continue to test for marijuana.

Medications

Many prescription and over-the-counter medications can impair driving: some antihistamines, sleeping pills, muscle relaxants, opioid painkillers, anti-anxiety drugs and some antidepressants among them. Labels often warn about drowsiness or operating machinery. Workers who drive should ask their doctor or pharmacist whether a medication affects driving, especially when starting a new one or changing the dose, and tell their supervisor if they are not fit to drive, without having to disclose the medical details. Employers should make it easy to report this without fear of discipline.

Other distractions and states of mind

Eating and drinking, smoking, grooming, pets loose in the vehicle, unsecured objects rolling around, and passengers who demand attention all distract drivers. So do strong emotions. Anger, grief and stress narrow attention and make aggressive driving more likely. Road rage often begins with a minor incident and escalates when both drivers treat it as a contest. A defensive driver lets the other driver go, avoids eye contact and gestures, and, if followed, drives to a public place such as a police station rather than home or to the workplace.

# Alcohol elimination is slow: rough illustration of BAC over time.
# Peak BAC for an example drinker after several drinks, then about 0.015 per hour elimination.
peak = 0.09          # illustrative peak in per cent BAC
rate = 0.015         # typical average elimination per hour; individuals vary widely
hours = 0
bac = peak
while bac > 0:
    flag = "  over 0.08" if bac >= 0.08 else ("  0.05 warn range" if bac >= 0.05 else "")
    print(f"After {hours:2d} h: about {bac:.3f}%{flag}")
    hours += 1
    bac = round(max(bac - rate, 0), 3)
print("Morning-after driving can still be impaired after a late night of drinking.")

After  0 h: about 0.090%  over 0.08
After  1 h: about 0.075%  0.05 warn range
After  2 h: about 0.060%  0.05 warn range
After  3 h: about 0.045%
After  4 h: about 0.030%
After  5 h: about 0.015%
Morning-after driving can still be impaired after a late night of drinking.

Passengers and co-workers

Passengers can help or hinder. A co-worker who reads the map, watches for signs and handles the phone can reduce the driver's workload. A passenger who argues, shows the driver things on a phone or distracts them with demands adds risk. Crews who travel together can agree simple rules: the passenger handles navigation and calls, nobody shows the driver a screen, and anyone can speak up if they see the driver distracted or drowsy. Speaking up is easier when supervisors have made it clear that it is welcome.