Trends in petroleum production, refining, and utilization can be expected to exert heavy pressure on middle distillate fuel supplies in the United States during the next several decades. The major consumers of middle distillate fuels are diesel engines that operate most long-distance trucks and railroad locomotives, as well as most agricultural machinery and gas turbine power plants employed by utilities in peaking applications. The increasing number of diesel passenger automobiles will tend to increase middle distillate fuel consumption. Although synthetic fuels from coal or oil shale could close any gap between potential supply and demand for middle distillate fuels, the noticeable lack of progress in construction of large-scale synfuels facilities does not offer as much cause for optimism now as it did a few years ago. Furthermore, the fact that petroleum prices have fluctuated instead of rising steadily has increased the risk of investing in huge synfuel enterprises.