A vehicle proximity warning system is being developed using GPS and wireless local area networks to improve safety of off-highway trucks in open pit mines. After two an a half years of research, software development, and laboratory testing, field tests were carried out at operating limestone quarries and open pit mining operations to evaluate GPS accuracy and overall system effectiveness under rugged operating conditions. This article describes the progress made on this NIOSH funded project and discusses the GPS accuracy tests and overall system tests that were carried out during 2001 at the Morenci open pit copper mine in Arizona.