The Hyperloop is a conceptual high-speed transportation system popularized by entrepreneur Elon Musk, incorporating reduced-pressure tubes in which pressurized capsules ride on a cushion of air that is driven by a combination of linear induction motors and air compressors. The conceptual route runs from the Los Angeles region to the San Francisco Bay Area, paralleling the Interstate 5 corridor for most of its length, with an expected journey time of 35 minutes, meaning that passengers would traverse the 354-mile route at an average speed of around 598 mph, with a top speed of 760 mph. A preliminary design document was made public in August 2013, which included an estimate of a US$6 billion construction cost for a passenger-only version of the system, while a version allowing for transportation of both passengers and vehicles was estimated at US$7.5 billion. The cost projections have been questioned by transportation engineers and others, who contend that the sum is unrealistically low given the scale of construction required and the level of unproven technology involved. The technological and economic feasibility of the idea is unproven and a subject of significant debate.