Multiple RNA polymerase II (RNAPII) molecules can transcribe a gene simultaneously, but what happens when such polymerases collide--for example due to polymerase pausing or DNA damage? Here, RNAPII collision was characterized using a reconstituted system for simultaneous transcription by two polymerases. When progression of leading polymerase is obstructed, rear-end collision entails a transient state in which the elongation complexes interact, followed by substantial backtracking of trailing polymerase. Elongation complexes remain stable on DNA, with their activity and the integrity of transcription bubbles remaining intact. Subsequent TFIIS-stimulated transcript cleavage allows resumed forward translocation, resulting in trailing polymerase oscillating at the obstruction. Conversely, if leading polymerase is merely stalled at a pause site, collision and TFIIS cooperate to drive it through. We propose that dynamic interactions between RNAPII elongation complexes help regulate polymerase traffic and that their conformational flexibility buffers the effect of collisions with objects on DNA, thereby maintaining stability in the face of obstacles to transcription.