Microjets, or very light jets (VLJs) are set to take off this year for the first time. With two engines, the planes seat about six people and cost half as much as the business jets now in service. With NASA predicting that 20,000 microjets will be in use by 2010, some are concerned that the addition of microjets will result in heavily congested airspace:

Unlike turboprops, which cruise below 30,000 feet, VLJs cruise at the   same altitude as jetliners – between 30,000 and 40,000 feet. But they cruise at   430 miles per hour, considerably slower than a 737, which flies at 500   mph.

Read Make Room for Microjets.