produce less noise than a conventional train at equivalent speeds. However, the psychoacoustic profile of the maglev may reduce this benefit: a study concluded that maglev noise should be rated like road traffic, while conventional trains experience a 5–10 dB "bonus", as they are found less annoying at the same loudness level. Magnet reliability: Superconducting magnets are generally used to generate the powerful magnetic fields to levitate and propel the trains. These magnets must be kept below their critical temperatures (this ranges from 4.2 K to 77 K, depending on the material). New alloys and manufacturing techniques in superconductors and cooling systems have helped address this issue. Control systems: No signalling systems are needed for high-speed maglev, because such systems are computer controlled. Human operators cannot react fast enough to manage high-speed trains. High-speed systems require dedicated rights of way and are usually elevated. Two maglev system microwave towers are in constant contact with trains. There is no need for train whistles or horns, either. Terrain: Maglevs are able to ascend higher grades, offering more routing flexibility and reduced tunneling. High-speed maglev comparison with aircraft Differences between airplane and maglev travel: Efficiency: For maglev systems the lift-to-drag ratio can exceed that of aircraft (for example Inductrack can approach 200:1 at high speed, far higher than any aircraft). This can make maglevs more efficient per kilometer. However, at high cruising speeds, aerodynamic drag is much larger than lift-induced drag. Jet-powered aircraft take advantage of low air density at high altitudes to significantly reduce air drag. Hence despite their lift-to-drag ratio disadvantage, they can travel more efficiently at high speeds than maglev trains that operate at sea level. Routing: Maglevs offer competitive journey times for distances of or less. Additionally, maglevs can easily serve intermediate destinations. Availability: Maglevs are little affected by weather. Travel time: Maglevs do not face the extended security protocols faced by air travelers nor is time consumed for taxiing, or for queuing for take-off and landing. Economics As more maglev systems are deployed, experts expect construction costs to drop by employing new construction methods and from economies of scale. High speed systems The Shanghai maglev demonstration line cost US$1.2 billion to build in 2004. This total includes capital costs such as right-of-way clearing, extensive pile driving, on-site guideway manufacturing, in-situ pier construction at intervals, a maintenance facility and vehicle yard, several switches, two stations, operations and control systems,