Watching the Highways
Not to detract from Schneier's take, let us roughly guess the relative frequency of "terrorist" events compared to ordinary everyday problems a motorist (including truckers) might encounter.
Looking at the real-time traffic information posted by the web, my guess would be there are more than 20 hazard events per day on the highways in an area containing less than a tenth of the country's population. Extrapolating that gets us a (very) rough estimate (rounded up) of about 100,000 hazard events per year for the entire country - just on the highways. Given that the number of terrorist events per year is something less than one, we are talking about a less than 1:100,000 chance of spotting a "terrorist" act compared to an ordinary hazard.
I do believe the Highway Watch program is generally an excellent idea. With less than 1:100,000 odds, seems that specific training to spot "terrorists" is pretty much a waste of time.