In 37 years at the General Motors Corp. Proving Ground (1934-1971), Stonex won international recognition for his contributions to roadside safety. Working with various highway agencies, he and other GM engineers designed and developed numerous improvements to reduce chances of death and injury to motorists, ranging from median safety barriers to break-away signs. He was a pioneer in "designing out" roadside hazards such as steep ditches and inadequate guardrails. As assistant director of the Proving Ground (1956-1971), he became a missionary for a safer roadside environment, traveling the United States and Canada to promote safety features, many of which are now written into state and federal highway standards.