Robert L. Kieckhefer of Silver Spring is intrigued by scientists who predict breakthroughs in medicine that will make it possible for people to live for 300 years.

He wonders how humans will react to the possibility of achieving such longevity.

Will a lad of 20 be more safety conscious when he has 280 more years of life at stake than he is today, with a chance for a "mere" 60 or 70 more years? Will he be more inclined to wear protective clothing -- for example, a safety helemt while riding his motorcycle? Will he be less inclined to jeopardize his health by smoking, or using drugs?

One day last week, Bob pondered these weighty questions as he drove toward a local shopping center. "A young man in a hurry" recklessly passed him on the right in a residential area, "narrowly missing parked cars to gain two or three seconds."

A few minutes later, Bob pulled into the Carrollton Mall and parked "out in a fringe area" because there didn't appear to be any closer spaces available.

After he had walked all the way from his parked car to the Woolco store, he noticed the young-man-in-a-hurry who had sped past him earlier. The impatient fellow was cruising up and down the parking lot aisles, searching stubbornly for a parking space close to the stores.

Bob wonders if the incident gives us a clue about the future.

Perhaps. It may indicate that future human conduct will be as irrational as present human conduct.

When our descendants are young, perhaps they, too, will live as if their term on earth will be endless -- and yet be willing to risk all eternity to save five seconds.

They may take these foolish risks in the mistaken belief that caution and prudence indicate a lack of courage. Science will offer them the gift of long life, but sometimes a moment of folly will rob them of it.

To those who remain alive to mourn the deaths of 20-year-olds, the loss of 280 years will be neither more nor less grievous than the loss of 60 or 70 years is today. After all, when one loses everything, what difference does it make how much everything amounted to?

Our best hope is that while most of those scientists are working on breakthroughs that will make our bodies last for 300 years, perhaps a few others will take a look at the wiring in our brains and try to make improvements there, too. Life gets too complicated when people are equipped with a Series 300 body and a Series 6 brain. THESE MODERN TIMES

It used to be rather commonplace for a man to remain on the same payroll for 40 years or more. Some who began at age 18 were still with the firm 50 or more years later, often as managers or even chief executives.

But the modern trend is toward much earlier retirement. Especially among public service employees.

For example: Among the thousands of members of the Metropolitan Police Department, not one has 40 years of service to his credit. The closest is Assistant Chief James M. Powell, who was sworn in on Nov. 1, 1940. And the only man who is within shouting distance of that mark is Sgt. Richard Latona, who completed 37 years this past summer.

The third-longest tenure is Sgt. William T. Thrower's. He has been on the force for 31 years and seven months.

Chief Burtell M. Jefferson is in fourth place with 31 years and four months to his credit.

Two deputy chiefs are the only others in the departments who were policemen here before 1950.

Deputy Chief Charles J. Corcoran joined up in June of '49, and Deputy Chief William C. Trussell took the oath in August of that year.

So only six of our policemen have more than 30 years of service. How times change! SIGN LANGUAGE

Al Brogdon writes from Damascus: "I liked your Army definition of 'Stop.' It reminds me of Air Force traffic signs I saw on Andrews AFB last month.

"As you drive through the gate, you see a sign that says, 'Speed Limit 30.' Then, about 200 feet farther down the road, there is another sign that says, '30 Means 30.'"

Al, your signs remind me of new ones that are appearing on California billboards. They show a picture of the ayatollah, and a four-word message: "Fight Back. Drive 55."

I wonder how many impatient young-men-in-a-hurry will be willing to sacrifice a few seconds of their precious time to join that crusade. ECONOMIC NEWS

"Food prices have gone crazy," says Bob Orben. "These days, a high roller is anybody who pushes a shopping cart. The worst thing about the war on inflation is that my savings are missing in action."