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You've booked a flight to the coast, but decide to drive when a commercial jet crash and two near misses occur in the week before your trip . . . .

You're at a convention in a large city, ready for a little nightlife after a long day. But others in your party convince you the streets are unsafe, so you spend the night in your hotel room having a few too many drinks instead . . . .You've been driving on the freeway several hours when you exit finally into congested city traffic. At these speeds, you figure you might as well slip off your seat belt and be comfortable . . . .

Though risks and chance surround us daily, most of us are poorly versed in the concepts of probability and statistics and how these bear on personal and public safety, says Dr. John D. McGervey, professor of physics at Case Western Reserve University in Cleveland and author of "Probabilities in Everyday Life" (Nelson-Hall Inc., Chicago, 1986).

According to McGervey, who admits to a lifelong fascination with numbers and games (he became a life master at bridge by age 22), most of us have a distorted sense of everyday hazards fostered largely by media headlining of dramatic events which leads us at times into thinking we're playing it safe when we're really taking unnecessary chances.

For instance, air disasters and near misses grab the headlines, but your chances of being killed on any given coast-to-coast commercial flight are only about one in a million, as compared with one in 8,000 when you go by automobile, says McGervey. "That means the risk is 125 times greater in your car, but how often do car crashes make the national news?"

And, of course, the risk picture for the car traveler darkens further if the driver is drunk, or speeds, or is at the wheel of a powerful car, or is smoking (one study of 105,000 policyholders by the State Mutual Life Insurance Company of America showed 2.6 times as many fatal accidents per mile for drivers so preoccupied).

"There's a natural tendency for us to downplay the risks in familiar situations or activities," explains McGervey. "Taking a plane is far more exotic, so it seems riskier than getting behind the wheel of your Nova or Toyota."

One safety factor that warrants special emphasis, adds McGervey, is the seat belt. He winces when he hears people tell him they buckle up only on long trips: The fact is, the fatality rate in city traffic is more than double that for freeways, and the most preventable of the nation's 50,000 deaths and 1.7 million injuries in motor vehicle accidents annually are those occurring in collisions at 30 mph or less.

"If 30 mph doesn't sound fast to you, imagine yourself running at top speed about 15 mph and crashing into a steel post. How do you think crashing at 30 mph would feel?"

And don't fall for the myth that seat belts are dangerous because they prevent you from being "thrown clear" in accidents, warns McGervey. Motorcycle riders are always thrown clear in mishaps. "Do you really think it might be better to become a human cannonball?" One Swedish study of over 28,000 accidents found unbelted occupants twice as likely to be injured or to die in accidents for all speeds up to 60 mph.

The physicist in McGervey leads him to contemplate the forces involved: If you're in a head-on collision at 30 mph, your unseatbelted body continues forward at 30 mph until it hits something: the windshield, steering column, door posts, etc. It's this second collision that does the damage _ no more preventable if you're not wearing a belt than is hitting the ground if you've fallen off a cliff.

In order to bring the passenger compartment to a stop from 30 mph in the .1 second of collapse requires a deceleration of about 14 gs. This means, explains McGervey, that if you're holding a 30-pound child in your lap, he or she will suddenly "weigh" over 400 pounds in your arms. "What are you chances of holding on to a child in an accident? Almost zero, judging from accident records."

Another common risk distortion that catches McGervey's attention is homicide. While gory details of street muggings or domestic violence get much play in the news, diseases take a hundred times the toll in human life. "Yet one study (pre-AIDS) of the print media found three times as many articles about homicide as about all diseases combined."

Yet while we tend to overestimate homicide as a cause of death (along with tornadoes, fires and floods), we vastly underestimate the risks of drinking and smoking, McGervey contends.

"Statistics show there are about 100,000 alcohol-related deaths in the U.S. every year, compared with 20,000 homicides. These include deaths by cirrhosis of the liver, accidents, suicides and others."

Figures from the National Cancer Institute indicate that the typical 25-year-old male who smokes two packs of cigarettes a day has a remaining life expectancy 8.3 years less than a non-smoker his age. Put another way, a 25-year-old male non-smoker's chances of reaching age 65 are 7 to 2 in favor, but only 3 to 2 in favor if he smokes a single pack a day. "And this added mortality risk applies even when the smoker is younger, and includes heart disease and others as well as cancer.

"A person is a lot more likely to be killed by drinking and smoking than by a mugger," McGervey sums it up.

One risk that people don't tend to minimize is radiation, says McGervey. In fact, a number of people canceled trips to Europe a couple of summers ago in the wake of the Chernobyl accident.

"Radiation levels in parts of Europe were 100 times higher than normal. Exposure to radiation at that level for one week might mean a person's chances of getting cancer would be 1 in 50,000," McGervey explains. "`Those who stayed home and took a 1,000-mile motor trip instead had the same chances of being killed in a car accident."

Radiation leakage from nuclear power plants in the U.S. has been calculated at an average of .003 rem per year _ enough to cause 100 to 200 deaths annually, says McGervey. By comparison, living in the mile-high city of Denver adds an extra .03 rem of exposure per year _ 10 times the plant leakage dosage _ due to more intense cosmic rays at that altitude (there's less atmosphere to filter them). Thus, a Denver resident experiences an extra one-in-a-million chance every two months of dying from cancer, calculates McGervey.

(Other one-in-a-million mortality risks include smoking 2 cigarettes or 1/20 of a "joint," driving 60 miles without a seat belt or 120 miles with one, motorcycling two miles without a helmet or five miles with one, or four minutes of flying in a private plane, McGervey reels off the statistics.)

"The purpose of comparisons of this sort isn't to scare people," insists McGervey, "but to put things in perspective. Life doesn't have to be so chancy if we're aware of the real hazards and balance off what's risked with what's to be gained."

Fortunately, a lot of chancy situations we confront aren't dangerous but involve such things as lotteries, life insurance or playing the stock market.

McGervey believes we could all do better for ourselves in these areas as well if we understood that the law of averages is folklore. "Luck has no memory. For instance, if a certain number has come up several times recently in a lottery, this doesn't mean it's less likely to come up next time to balance things out. But that's the way most of us tend to look at it."

Insurance, too, is really a kind of gamble, McGervey points out: You bet the insurance company something bad will happen, the company bets it won't. "This is one bet you usually want to lose."

He warns against confusing insurance with savings. Spend what you need on insurance, then invest the rest in a suitable savings plan.

"Insurance companies may try to disguise the fact that true insurance is a bet, but if it isn't a bet, it isn't insurance."

The stock market can be viewed as a glorified casino, but is often not as good as gamble as the Las Vegas variety, McGervey contends. "To come out ahead on Wall Street, you not only need to pick a winner but an underpriced winner. Playing the stock market is even tougher than, say, betting on horses, because you never know when the race is over. It ends when you've had enough, or run out of money."