Incidents in which a piece of construction equipment backed into a worker resulted in an average of 17 deaths per year at road construction sites and 15 deaths per year at building construction sites from 1997 through 2001. This trend continues and researchers at the National Institute for Occupational Safety and Health are evaluating methods to decrease these incidents. A new technology based on the detection of electronic identification tags worn by workers has been developed and evaluated at a road construction site. The tag-based proximity warning system consists of a magnetic field generator and communications system that mounts on the back of a piece of construction equipment such as a dump truck, road grader, or loader. Workers at a construction site wear a small tag that detects the magnetic marker field. If the electronic tag is within 15 ft (4.6 m) of a reversing piece of equipment, an audible alarm is sent to the equipment operator, and an audible and vibrating alarm is generated by the worker's tag. Preliminary tests show that this cooperative two-way warning system is effective in detecting workers near reversing equipment and may have potential to decrease backing incidents in certain work environments. The system's limitations are identified and recommendations for future improvements are discussed.