If you've been driving a car since your teen years, it probably seems like second nature to you by now. But in reality, driving is one of the most complicated tasks a human being performs on a regular basis.

When you drive, you must constantly keep track of a vast array of input about what's happening on the road, interpret that information, and use it to update your decisions about how to maneuver the vehicle.

Driving requires your visual and hearing systems, spatial perception, working memory, ability to plan, reflexes, concentration, and many other neurologic capabilities.

"Many neurologic conditions involve impairments in functions that are essential for driving, like perception, attention, decision making, and critical response time," says Matthew Rizzo, MD, Fellow of the American Academy of Neurology (FAAN) and chair of neurological sciences at the University of Nebraska Medical Center. "The more impairments you have, the more likely you are to commit a potentially dangerous error while driving," he says.

But having a neurologic condition doesn't necessarily mean giving up the car keys. Some people with well-controlled epilepsy, or early-stage Parkinson's or Alzheimer's disease, might be perfectly capable of driving. "For many of these diseases, however, progression will lead to more and more problems with driving. There's no way around it," Dr. Rizzo says.

Let's take a look at four common neurologic conditions and how they can affect driving.

Epilepsy

Epilepsy is unique among neurologic conditions in that every state in the country has laws defining it as a condition that limits the ability to drive. "You cannot drive if you have active seizures that limit the ability to drive, typically defined as a period of loss of consciousness," says Joseph Drazkowski, MD, FAAN, professor of neurology at the Mayo Clinic in Phoenix, AZ, and an expert in driving and other quality of life issues for people with epilepsy.

Different states require different seizure-free intervals before someone is allowed to drive. Some states have no specific mandatory seizure-free period but instead rely on physicians' judgment. (To find your own state's laws, search the Epilepsy Foundation's database.) "Minnesota recently went from a law requiring a 12-month seizure-free interval to one requiring only three months, so it's now one of the most liberal states in the country," says Dr. Drazkowski. His own research has found no significant increase in crashes with a required seizure-free interval of three months compared with a longer period.

Of course, the risk of experiencing a seizure is only one aspect of having epilepsy that may interfere with safe driving. Many people with epilepsy take one or more medications that may, either individually or together, impair reflexes, response times, and other functions important for driving. "If you're on high doses of Dilantin, for example, you may look like you'd fail a sobriety test. Your eyes jiggle and you're unsteady walking," says Dr. Drazkowski. "I've had to tell patients on a few different medications, such as this one, that they're unsafe to drive. You can't really make it one size fits all."

Alzheimer's Disease

You might assume that someone who has been diagnosed with Alzheimer's disease (AD) definitely shouldn't drive, but that isn't necessarily the case. "Patients with mild dementia are overall considered to be higher-risk drivers, but studies have found that as many as 76 percent of people in the early stages of the disease are able to pass a road test and can still drive safely," says Donald Iverson, MD, FAAN, a neurologist with the Humboldt Neurological Medical Group in Eureka, CA, who helped develop the AAN guideline on driving and dementia. "Mild forgetfulness doesn't mean you're not safe to drive."

But unlike epilepsy, AD is invariably a progressive condition. At some point, a person with dementia will no longer be safe on the road. Some signs of declining driving capabilities include:

• Driving well below the speed limit
• Having difficulty with lane changes or intersections
• Giving up driving at night or in bad weather
• Aggressive or impulsive behavior behind the wheel
• Accumulating traffic tickets or fender benders
• Getting lost

"Giving up the keys to the car means giving up a lot of freedom, and it's a hard decision to make," says Dr. Iverson. And because dementia, by its very nature, robs a person of their mental capacity, it may not be a decision that the person can make himself or herself.

Since driving ability often declines fairly rapidly among people with AD, the AAN recommends reassessments every six months for people with dementia who continue to drive.

Only a handful of states currently have laws that specifically address dementia and driving. In California and Pennsylvania, health care providers are required to report all patients diagnosed with AD to the Department of Motor Vehicles (DMV). Oregon law requires that certain physicians and health care providers report patients with "functional and/or cognitive impairments that are severe and uncontrollable" to the DMV, but its Web site specifies that this does not mean that all persons with dementia must be reported.

Many other states don't mention AD or dementia specifically, but their laws either ask or require physicians to report people with conditions that might impair their ability to drive safely. (The AAN supports voluntary physician reporting, but believes that mandatory reporting of Alzheimer's or any other condition could compromise the physician-patient relationship and lead patients to withhold important information from their doctors.)

The Alzheimer's Association offers a dementia and driving safety guide to help ease the transition.

Parkinson's Disease

Most of the primary symptoms of Parkinson's disease (PD), such as tremors, stiffness, and slowed movement, can cause problems driving a car. PD can also cause impairments in mental function, such as slowed reaction times and problems with visual perception and memory. And common PD medications, such as drugs to regulate dopamine levels, can cause daytime sleepiness.

But research has found that, as with AD, some people who have early-stage PD may still be able to drive safely. In a study published in the journal Movement Disorders in December 2013, certified driving assessment experts rated the driving skills of 104 active, licensed drivers with PD. They found 65 percent competent to drive, while 35 percent failed the road test.

A German study published in the AAN journal Neurology in December 2013 found that deep brain stimulation (DBS) may benefit driving ability for people with PD. People with PD who had DBS drove more slowly than healthy control volunteers but otherwise, their driving was about as safe, while patients who had no DBS made significantly more driving errors than either the control group or the DBS group.

None of the state laws that govern driving and medical impairments mention PD specifically, but laws that require doctors to report patients with conditions "that could impair his or her ability to safely operate a motor vehicle"—an example from Pennsylvania—to the DMV would probably be considered to include PD.

Multiple Sclerosis

Unlike AD and PD, multiple sclerosis (MS) often strikes younger people in the middle of their active career and parenting years, when the inability to drive can pose even more of a problem than it would for someone in later life.

When MS flares up, many of its symptoms can make driving hazardous. Optic neuritis can cause visual disturbances. The stiffness, numbness, and decreased sensitivity in the hands and feet can interfere with the ability to control the steering wheel, the brake pedal, and the accelerator. The fatigue and muscle weakness that are common with MS can also impair driving.

Fortunately, most people with MS—especially those who go months or years between exacerbations—can continue to drive safely, although they may have to make arrangements to avoid driving when flare-ups occur. As the disease progresses, however, physical symptoms may worsen, and cognitive changes may make it harder for the person to detect when they can no longer safely drive. Many drugs that treat common MS symptoms such as spasticity and neuropathic pain can also slow reaction times and impair judgment.

Am I Safe or Not?

"The disease itself might make you suspect there's a problem with driving, but often a diagnosis is insufficient to determine whether you're safe or unsafe," says Dr. Rizzo. That's where testing comes in.

"It's hard to know when to stop a person from driving based on the diagnosis alone," says Dr. Rizzo. "Doctors may suspect that someone is no longer able to drive safely, but they're not trained to assess driving skills. The best way to determine if you, or someone you love, can safely continue to drive is with a professional driving skills assessment."

The American Occupational Therapy Association has a database of driving programs, with trained specialists who conduct "driver rehabilitation" tests designed to identify a driver's strengths and challenges and determine whether they can drive safely. The tests can also determine what modifications they might need, such as a car with hand controls, or if any restrictions should be made on when they drive, such as only during the day, in good weather, or on familiar roads. You can also find resources through the Association for Driver Rehabilitation Specialists.

If you have a family member with a neurologic condition that may impair driving, ask yourself: would you want to be on the road with this person, or would you let your children ride with them? "If you have a sense that they're not safe, they're probably not," Dr. Iverson says.

If it's time to think about giving up the car, that doesn't have to mean drastically reduced mobility. There are many other options for getting around besides driving your own car. 

The prospect of losing the ability to drive is extremely upsetting for many people. "It's life-changing," says Dr. Iverson. "Giving up the keys means an enormous loss of independence. It's nothing to be cavalier about. But for many people it does end up being a reality, not just for those with neurologic conditions but for other physical and mental infirmities as well."

Tests to Determine Whether Driving Is Safe for Neurology Patients 

• NEUROPSYCHOLOGICAL TESTING. You may have taken some of these tests in the process of getting a diagnosis for your condition. These standardized ways of measuring higher cognitive and perception abilities can help to identify functional problems that may interfere with your ability to drive safely.
• A DRIVING SIMULATOR. It's kind of like an arcade video driving game, except it evaluates the moves you make on the "road" and gauges your driving skills. Some of these simulators are stand-alone machines, while others can be used in a real car—sometimes even your own. Because they're still not the real thing, simulators shouldn't be the final word on driving safety, says Dr. Rizzo. "But they can be used as a screening tool to see if it's safe for someone to do a road test."
• ROAD TESTS. On-road testing in a real-world environment, with a trained specialist in driver evaluation, is the best way to find out if you can safely continue to drive.

Alternatives to Driving

• Public transportation. Subways, buses, and light rail can be good options in areas that have them—usually larger cities. Many have reduced prices for older adults.
• Ride sharing. Ask family, friends and neighbors if they can help you get around. You can offer to pay for gas, or contribute in other ways like baking goodies for them.
• Senior transit options. Many communities have designated shuttle services for seniors. Hospitals and houses of worship sometimes offer their own. Check with your city's public transportation office or your local Office on Aging.
• Taxis. These are particularly useful for unscheduled trips with short notice. You can also check out taxi-alternative services like Uber and Lyft, now in dozens of cities nationwide.
• Volunteer service organizations. Ask about volunteer drivers at your local community center, YMCA, or other social service groups.
• Support groups. Check with support groups for people with your neurologic condition; sometimes they have networks of volunteers available to help with different needs, and that may include transportation.
• Easter Seals Project Action promotes universal access to transportation for people with disabilities. Their Web site is a treasure trove of resources that can help you get around.
• The National Aging and Disability Transportation Center provides grants to volunteer transportation programs around the country. Check to see if there's one in your area!