There were a number of factors that contributed to the drop in gasoline prices in 2014, most notably the fact that the United States experienced an boom from 2008 onward in shale oil; however, additional factors, such as increased stability in oil-producing countries like Libya, led to an increase in global oil production. Interestingly, despite the drop in price, costs associated with locating new oil reserves continued to increase.
2014 also saw Organization of the Petroleum Exporting Countries members engage in a price war in order to retain as much market share as possible. Saudi Arabia and Kuwait in particular pushed oil prices lower in order to attempt to keep hold of market share, which ultimately proved beneficial for oil consumers around the world.
Less demand from Asian countries is also likely to have had an impact. Slow economic growth led many Asian countries to force up fuel costs by slashing subsidies. This led to much lower overall consumption of fuel, diesel in particular. This lowered demand caused prices to drop.
Huge growth in American oil production also lessened demand for oil imports, reducing the value of oil produced in countries such as Nigeria, who were previously a major oil exporter to the U.S.
When combined, these various events and economic climates pulled down the price of gasoline for most countries in the world, particularly large consumers like the U.S.