It was not a mistake made by highway builders, nor was it ever intended to be a bridge. This structure was actually built as a test track for experimental mobile radar units on Grand and Normandy Avenues in Chicago. Western Electric built it in 1943 to test and tweak their mobile radar equipment, which was new cutting-edge technology.
The area around Grand Avenue and Fullerton is the highest point in Chicago which is located on a natural ridge. The location was few blocks away from where Thunder Mountain, Chicago’s only ski resort chose to build on it in 1967. The high elevation and 40 foot above ground track kept the equipment clear from ground echoes.
Mobile radar units would be driven up a wooden ramp to get onto the track. Airplanes from Glenview Naval Air Station would fly over the track allowing the radar units to collect data. Later, the Navy would evaluate the data and the equipment’s efficiency.
Radar was at the time a new technology that wound up being crucial to American success in World War II. The track was used throughout WWII and the Korean War being crucial to American's military success.
After the wooden ramp was removed, the track sat unused for more than 40 years because it was too costly to demolish it. Finally, in the 1990s, it was razed to build a new strip mall.
WTTW Chicago - wttw.com