I slow and flash my lights to yield space and give courtesy to drivers all the time. It removes the uncertainty so the other driver knows if he should slow down just in case, or do his best to gain speed for merging- knowing he has the room to merge.
This is even more important with larger vehicles that have limited acceleration and stopping capabilities.
Someone hitting you from behind is presumed at fault with the possible exception if the front driver actually SKIDS his tires or is "brake checking"
Courtesy and safe operation is always the right answer, as opposed to "I am driving whatever way I want and everyone else has to maneuver around me."