During an emergency response to a roadside accident site, the helicopter experienced an in-flight collision with a tree that resulted in a main rotor strike. The Director of Flight Operations reported that the pilot had chosen a landing spot that had a tree on the southwest (SW) side of the road and a tree on the northeast (NE) side of the road. As the pilot came to a hover over the landing site, he was looking left and forward up the road to confirm that the area was secure. As he was looking toward the left front, he heard a discernable noise change from the rotor system, although there was no change in aircraft handling or any abnormal instrument indications. He looked to the right and observed that he was closer to the NE tree than he wanted to be. He then observed a broken branch on the NE tree. After a normal shut down, a visual inspection showed damage to the outer three to five inches of the trailing edge of the three main rotor blades.