If you should be on the highway when "42nd Street's" 19-tractor-trailer caravan is en route, watch out. The show is like a small city, traveling with its own beauty salon, washing machine and dryer, and tailor shop, not to mention more than 2,000 wardrobe items and 20,000 light bulbs, all of which are installed in whatever theater the show is playing.

There are four companies of the show now tapping away, which should qualify it as a minor industry. Two are more or less permanently playing in London and New York, and the other two are on the road. It takes a week just to install the 20 sets -- or 40 people working 120 hours. To facilitate matters, each road company has two floors, or decks, that alternate between cities. While one floor is being tapped on in Washington, the other floor is being installed in Philadelphia. Or vice versa.

Three veteran David Merrick employes are the generals in charge of this army. Willard Shaffer, who has worked for the producer for 30 years, coordinates the tractor-trailers and supervises the put-in and the strike. Fred Feller, who has been building sets for 42 years, 30 of them with Merrick, is the "advance man," and handles the specifics of assembling the sets. Leo Herbert is called production property man, but he actually figures out where to house the costumes and props in each theater, sets up places for quick costume changes and worries about moving the 71 company members and all their personal stuff. A former actor, Herbert has been in technical theater since 1947.