During a flight test flight, the pilot applied full power and the airplane became airborne in 400-500 feet; the tachometer read 2,240 instead of the planned 3,000 rpm. The pilot continued and the initial rate of climb was "...low but positive." The airplane climbed to approximately 100 feet, and when at midfield, the flight encountered turbulence which resulted in a loss of altitude (10-20 feet), and a loss of airspeed (several miles-per-hour). The pilot recovered both and a positive rate of climb was re-established. When approximately 3/4 down the runway, the flight again encountered turbulence resulting in the inability to maintain a positive rate of climb. The pilot lowered the nose of the airplane in an effort to avoid a stall, but the airplane collided with a concrete berm near the departure end of the runway. The airplane then impacted the ground adjacent to a 4-lane highway, and the airplane and a vehicle on the road collided. The pilot reported there was no preimpact mechanical failure or malfunction of the engine, flight controls, or systems.