On August 2, 2016, at 1830 central daylight time, a Cessna 172K, N46459, collided with a road sign during an off airport forced landing on Highway 10 in Neenah, Wisconsin, following a loss of engine power. The private pilot and two passengers were not injured. The airplane received substantial damage to the left wing. The airplane was registered to and operated by the pilot under the provisions of 14 Code of Federal Regulations Part 91 as a personal flight. Visual meteorological conditions prevailed for the flight, which was not operated on a flight plan. The local flight originated from the Brennand Airport (79C), Neenah, Wisconsin, at 1815. The pilot reported that he owned another airplane, an RV7 that was equipped with a Jabiru 5100 engine. The vacuum pump on that engine recently failed so a mechanic removed it until a new one was received. The vacuum pump on the Jabiru 5100 is fitted to the alternator mounting flange and the drive pad is dry. The pilot was told that he could fly the airplane without the vacuum pump installed which he did uneventfully for about 6 hours. While flying the accident airplane, a Cessna 172, three days prior to the accident, the pilot noted the directional gyro and attitude indicator where not working. Because of the past experience with a vacuum pump failure on the RV7, he identified the problem as being a vacuum pump failure. The pilot held a Repairman Experimental Aircraft Builder certificate for the RV7, and stated that because he was comfortable working on that airplane, he decided to remove the vacuum pump from the accident airplane himself. He did not install a cover plate over the vacuum pump mounting pad. In addition, since he was able to fly the RV7 without a vacuum pump, he believed he could also fly the accident airplane without a vacuum pump installed. During the first flight after removing the vacuum pump, the airplane sustained a loss of engine power. The pilot landed the airplane on a highway, and while traveling up an exit ramp, the left wing contacted a road sign. The airplane spun around and came to rest in a ditch. A postaccident examination of the engine revealed the engine suffered oil exhaustion when the oil exited the engine through the vacuum pump mount in the accessory case.