Two people are dead after their motorcycle crashed on I-55 near the Stevenson exit in Sangamon County.   The Illinois State Police report that as traffic was coming to a halt Sunday due to an earlier collision, a motorcyclist lost control and slid onto the left shoulder.  The male driver, 48-year-old Mitchell Bledsoe of Normal died at the scene and the female passenger, 35-year-old Karla Blake died at an area hospital.