U.S. safety investigators, concerned about rollover crashes, have asked regulators, as well as General Motors Corp. and Ford Motor Co., to take steps to improve the safety of 15-passenger vans, according to a Reuters report. The National Transportation Safety Board wrote to the chief executives of both companies Nov. 1, recommending the manufacturers look into technology that would help drivers maintain better control of these vehicles, Reuters said. According to highway safety data, large vans are involved in a higher number of single-vehicle accidents involving rollovers than are other passenger vehicles, Reuters reported.