Accident-prone persons may be relieved to learn the odds against being run over by a snowplow are steadily improving. But lest they develop a false sense of security they also should know the chance of becoming a golf cart casualty will increase ominously in the new decade. This whimsical good-news-bad-news outlook can be found among the statistical projections in the January issue of American. Demographics, a magazine devoted to the study of population trends. Anticipating the 1980 census, and the myriad uses that can be made of the data it harvests, the publication notes that population changes influence both the character of man-made calamities and the extent of havoc wrought by natural disasters. If, for example, the population of Sicily dropped sharply, there would be a corresponding drop in the statistical probability of any given person becoming a victim of volcanic eruptions. As American Demographics puts it, with tongue in cheek: "When we match demographic trends with regional disasters we can begin to see the dim outlines of the science of demodoom, which is the correlation between population and ruination." The upcoming census is expected to show a population in which the average age is increasing and that is shifting toward the so-called Sun Belt. Both trends will be detected in the accident reports of the 1980s, the magazine predicts. "The migratory goal of many Americans is southern California, an area of great seismic activity. As population density and geological upheavals are juxtaposed, we find an increasing likelihood that any given American will be confronted with an earthquake. "But because the population is older, they will be more likely to live on the lower floors of tall buildings and less apt to do something foolhardy such as running out into the street." Meanwhile, "with legions of older Americans flocking to gold courses and unwilling or unable to carry their own clubs and being unable (also for demographic reasons) to find an able-bodied teenager to caddy for them, there will be record swarms of electric golf carts of the fairways." The upshot: "Fewer people will be struck by snowplows" but record number of golf cart accidents is in prospect. Furthermore, the magazine adds cheerfully, there will be less exposure to such "inherently dangerous winter sports" as ice fishing and marshmallow roasting.