U.S. crude prices plunged to their lowest levels in 18 years on Wednesday, as governments tightened travel restrictions across the world and the continuing Saudi-Russian price war showed no signs of abating. West Texas Intermediate futures—the main U.S. crude gauge—dropped 24% to $20.37 a barrel, hitting their lowest level since February 2002. Brent crude, the global benchmark, fell 13% to $24.88 a barrel, its lowest level since May 2003. …