Crude prices have plunged by more than 20% in recent months. The fall means cheaper gas, cheaper flights, and generally good news for American consumers. Oil prices have been falling for months, and the slide could easily continue. With unprecedented instability in the Middle East, how could this be happening? The answer, according to analysts who follow the market, is a little bit of everything, from economic fundamentals in big oil consuming countries to growth in output among some oil producers. But much more clear is the effect of tumbling prices: Low oil prices are the equivalent of a massive...