July 09, 2004

Exit only this lane

Jeff Erickson writes to me from the University of Illinois, apropos of my recent post on exit signs, to remind me (for I had also noticed this myself) about the freeway signs that say EXIT ONLY THIS LANE. Careful observation of lane markings and thought about the logical form (best done when travelling as a passenger) will reveal that sometimes they are clearly intended to have a meaning in which ONLY goes with EXIT ("exit only, if this lane is the one you're in") and sometimes they definitely have a meanin in which ONLY goes with THIS LANE ("only this lane, if exiting is what you want to do"). Jeff adds that sometimes you can read the sign as the conjunction of those two meanings ("only exiting is allowed from this lane AND only from this lane can you exit"). How could gross and consequential ambiguities like this be tolerated in an environment where, to an almost unique extent, accurate reading and parsing is essential at high speed and life and limb may depend upon the interpretation process? All I can add is that we're just lucky that they don't (as far as I have seen) tend to write such signs on the road surface. If they did, you'd be driving over something that looked like this:


Posted by Geoffrey K. Pullum at July 9, 2004 01:46 PM