The driver of a vehicle approaching a yield sign shall in obedience to such sign slow down to a speed reasonable for the existing conditions and shall yield the right-of-way to any vehicle in the intersection or approaching on another highway so closely as to constitute an immediate hazard during the time such driver is moving across or within the intersection; provided, however, that if such a driver is involved in a collision with a vehicle in the intersection, after driving past a yield sign without stopping, such collision shall be deemed prima facie evidence of his failure to yield right-of-way.