An insidious monster stalks our nation's highways, threatening death and injury to those who venture out of their driveways for the drive to work, the errand to the supermarket or the vacation trip of their dreams.

The monster is "speed creep," the offspring of drivers who persist in going 5 to 10 miles above any posted speed limit. The latest example has occurred in New Mexico, the first state to raise the speed limit from 55 mph to 65 mph on rural interstate highways. In 1985, 47 percent of motorists violated the 55 mph limit. Just a week after New Mexico raised the limit to 65, only 37 percent exceeded it. Two months later, however, the number is 49 percent -- more speeders than with the 55 mph limit.

But speed creep lives not only on the rural interstates. Drive up I-270, down Canal Road, out New York Avenue. Try Georgia Avenue or Connecticut Avenue in Maryland or Lee Highway or Rte. 1 in Virginia. Or just drive through your neighborhood. Whatever the limit, drivers add a few miles per hour.

What are we to do? First, we must recognize that motorists will slow down in areas where enforcement is frequent and consistent. Second, we should encourage more enforcement of existing laws. And third, we must realize that any time we increase speed limits, many individuals will continue to drive faster than the posted limit.

Speed limits exist for the common good; they should be obeyed and enforced to serve that common good. TONY NICELY Great Falls