As the weekend voice of Shadow Traffic in the 1980s, John Russell Ghrist monitored highway snarls from a telescope on the 94th floor of the Hancock building, broadcasting the bad news for WLS and WMAQ. Now he's a different part of the traffic equation, dispensing public information for IDOT. But on Saturday afternoons he's in the studio at College of DuPage producing Midwest Ballroom, an hour of big band music played by contemporary local musicians, on WDCB. He started the show three years ago when he realized "a lot of people go to ballrooms like Glendora, Willowbrook, and Chevy Chase to hear the big bands, but they're not getting any airtime. With about 80 radio stations in Chicago, I'm the only one playing them." Ghrist says 50 area bands play the old music, and that many of them give it an original spin. This week he'll be airing cuts of Glenn Miller music from the CDs of groups like the Vito Buffalo Orchestra, the Yorkville American Legion Big Band, and the Airmen of Swing--who, he says, sound just like Miller's orchestra. It's part of a seven-hour Glenn Miller special produced by Ghrist with WDCB's other Saturday DJs. The afternoon will also include Ghrist's interview with Marvin Negley, president of the Glenn Miller Birthplace Society in Clarinda, Iowa, which hosts an annual festival in June. Miller, who had the most popular band in the world when his plane disappeared over the English Channel during World War II, was born 100 years ag0 this month. The special runs from 11 to 6 on Saturday, March 6, at 90.9 FM or at www.wdcb.org; Midwest Ballroom is the final hour.