Bridge-construction practices and reflective tape on semi-trailers

Mr. Alice:

Whenever I encounter bridge construction on the interstate, or most any other highways and roads, fastened onto the temporary supports at both sides of the road is a painted and illuminated, white board of significant size. What is it for? Since I’m imposing on your generosity, what’s with the inverted, L-shaped strips of reflective tape at the top rear corners of semi-trailers?

— Geezer, Escondido

I’ll overlook the fact that you’re double-dipping into the Alice Fountain of Knowledge on this one, Geezer, thus proving the depths of my so-called generosity. The white boards in question serve as a kind of light control. They prevent the construction site’s lighting from spilling out and causing glare, focusing it on the underside of the bridge where it’s most needed in the perpetual half-darkness of the highway overpass. This (rather brilliant) idea to use them came about after decades of motorists complaining to Caltrans about being blinded along the freeways.

As for the reflective strips on semis, they’re for taking digital height measurements at weigh stations and such. Retro-reflective materials (which you may recall from my column last week) are useful in measuring distances because lasers bounce off of them with uncanny precision. The Apollo astronauts actually left a battery of retro-reflective markers on the surface of the moon for use in calculating the distance between Earth and Moon with amazing accuracy. Compared to that, measuring the height of a tractor trailer is a piece of cake.