State Route 12 Mass Casualty Accident Results in Five Injuries. A major casualty accident was reported near Isleton, east of Rio Vista, on January 19 that involved two vehicles. The collision happened along eastbound State Route 12 just east of Jackson Slough Road around 12:44 p.m. The California Highway Patrol (CHP) reported that one of the vehicles, a Jeep, was found overturned onto its roof.