July 08, 2004

Left turn only

I have often wondered whether road lane signs with ONLY under a left-bent arrow mean that you can only turn left from that lane or that the only lane you can turn left from is that one. It seems to me dangerous to have to ponder a tricky scope problem, on which life-or-death lane-changing decisions may hang, while driving in heavy traffic. But it is particularly interesting that in 1971 the State of Florida made a mistake about it on a driver's license exam. The question showed a sign like the one shown here, and it asked what the sign meant. The correct answer was supposed to be:

Left turn from left lane only and traffic in adjoining lane may turn left or continue straight ahead.

But almost everyone reads this as a contradiction. Don't you?

The person who wrote the question and its incorrect answer appears to have been one of the minority who utterly confuse "Only if you're in the left lane can you turn left" with "If you're in the left lane the only thing you're allowed to do is turn left." The error was spotted by John Keasler, who wrote about it in the Miami Herald on November 23, 1971, page 8-B. Howard Pospesel gives the task of stating the two meanings explicitly in logical symbols as an exercise on page 61 of his textbook Introduction to Logic: Predicate Logic (second edition; Upper Saddle River, NJ: Prentice-Hall, 2003).

Posted by Geoffrey K. Pullum at July 8, 2004 09:25 PM