A device that uses ultrasonic sensors to detect objects in selected spaces is being installed in transit buses to keep the rear service doors from accidentally closing on passengers. The device, manufactured by Vapor Bus International, is called CLASS™ (Contact-Less Acoustic Sensing System). It uses three sensors and a signal-processing module to determine whether an obstacle is present in the doorway. The Orange County Transportation Authority (OCTA) in Orange, Calif., has installed CLASS in 20 of its articulated buses and eight standard 40-foot buses in an effort to reduce the risk of door injuries to passengers. “It’s especially important with senior citizens and fragile people,” said OCTA spokesman George Urch. The OCTA began testing the Vapor system in 1999 after attempts to find a workable solution with a different device, which used sonar sensors, failed because of temperature and humidity sensitivity. “The bus manufacturer worked with the manufacturer of the device for three years, but couldn’t get it to work properly,” Urch said. Urch added that the CLASS system is still being scrutinized by the OCTA. “We’re continuing to test it and, if it works out well, we may retrofit the rest of our buses,” he said. John Condon, national sales manager at Vapor, said the CLASS system is also being tested at transit properties in New York; Toledo, Ohio; Chicago; Portland, Ore.; and Kitchener, Ontario.