On April 24, 2004, at approximately 1140 mountain daylight time, a Cessna 172N, N5048E, was substantially damaged when it struck a sign during a forced landing near Lyons, Colorado. The commercial flight instructor and pilot receiving instruction were not injured. Visual meteorological conditions prevailed. No flight plan had been filed for the local flight being conducted under the provisions of Title 14 CFR Part 91. The flight had departed Vance Brand Airport, Longmont, Colorado, at approximately 1050. According to the flight instructor, they were practicing engine failure procedures (with the use of carburetor heat) when the engine lost power. He made a forced landing on a road near Lyons, Colorado. During the landing roll, the airplane struck a street sign. The right auxiliary wing spar was bent, the right leading edge was crushed, and the left horizontal stabilizer was crushed. The engine was run by an Airframe and Powerplant mechanic on April 24, 2004. The engine started without hesitation and a carburetor heat check and magneto check were performed with no anomalies. The fuel tanks were reported to be "half full." The Fort Collins Loveland METAR (Routine Aviation Weather) taken at 1055 reported the temperature as 45 degrees Fahrenheit (F), and the dewpoint as 33 degrees F. According to the Carburetor Icing Chart, the conditions were conducive for "serious icing at cruise power."