The nation's highways just keep clogging up, according to the annual Reason Foundation study that measures each state's road conditions and expenditures. The Reason Foundation's 17th Annual Report on the Performance of State Highway Systems, released in July, measures the condition of all state-owned roads and highways from 1984 to 2006. According to the study, California has the worst traffic congestion: 83 percent of its urban interstates are congested. But other states are becoming increasingly gridlocked too: 18 states report at least half of their urban interstates are jammed.