I run a small mechanical contracting firm in NoVa.  I have numerous vans and trucks (14) which may have no rear windows (to keep van from being broken in to) but mirrors are a must!!  Do the mirrors satisfy the regulation for my existing vehicles?  Most of the guys work and travel alone, and to have a second person for flagging is usually not realistic.  My guys can be trained to walk and look before backing, but this is not foolproof.  To retrofit all of the trucks with backup horns is not economically feasible in the short term (cash flow issues).  Would beeping the car horn be sufficient for a back up alarm?  KG