</font><font color="blue" class="small">( "I'm not aware of any jurisdiction (In the U.S.)where "travelling below a certain speed requires the use of hazard lights""
Pennsylvania and Maryland do...if the vehicle is moving below a certain speed (40 MPH on a highway, for example). )</font>
I think I've seen the signs on the Interstate in the Northeast that say "minimum speed 40 MPH". I would think if your going to go less than the minimum posted speed, your a hazard and you should pull over.
If thats the law then it's not well thought out. If there is a driving rain, and ALL the vehicles slow to under 40 MPH, and ALL the vehicles turn on their flashers, everyone is driving into a Christmas tree. Nobody can tell who is braking, signalling a lane change or just cruising with their hazards on. I respond to crashes daily in the rainy season and hear people all the time say "I didn't know he was braking...he had his flashers on". True, but the problem is he is following too close which doesn't leave enough assessment (of the flashing lights) and reaction time.
I think the circumstance of a heavily loaded semi, climbing a hill, under the posted speed limit is a good use of hazard lights. My concern is MASS use of hazard lights, primarily in DRIVING rain. It's dangerous and distracting. Driving slow, with head/tail lights and appropriate spacing (2 second rule... minimum) allows for better concentration on the surrounding vehicles without the constant, unnecessary assessment of dozens or hundreds of "flashing lights".
The human eye is attracted to movement as well as a flashing light. It's natural. Conciously or not, you ARE assessing each of those flashing lights for what they MAY be...Brake lights. And while your brain is pretty fast, it's not fast enough to assess ALL those lights in time to prevent a collision.