Or. Admin. R. 437-007-0525
All vehicles must be equipped with a horn or audible warning device which can be clearly heard above the surrounding noise in the vicinity of the vehicle.
ORS 654.025(2) & 656.726(4)
ORS 654.001 - 654.295
OSHA 5-2003, f. 6-2-03, cert. ef. 12-1-03