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I know that restrictions do not always apply and ATC can cancel them or pilots can request for speed restriction being canceled. But why is there such a speed restriction in first place? Are there any reasons other than noise abatement?

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    $\begingroup$ Because in the areas were aircraft need to operate below 10;000 ft, the density of traffic is higher. $\endgroup$ – Simon Sep 10 '15 at 19:03
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The speed limits were set by FAA in the early days of jet age for better control of the aircraft and safety where the air traffic (in VFR) is the densest and have little to do with noise.

FAR § 91.117 states

Unless otherwise authorized by the Administrator, no person may operate an aircraft below 10,000 feet MSL at an indicated airspeed of more than 250 knots (288 m.p.h.).

TWA Flight 553, a DC 9 collided with a Beechcraft Baron, killing 26 people on March 9, 1967. NTSB investigation determined that due to the high rate of descent of the DC-9, its pilots were not able to see the other plane in time to avoid a collision.

Following this accident the FAA issued Advisory Circular 90-32 titled 'Air traffic control and general operations, radar capabilities and limitations' in Aug 1967.

According to NTSB report on the accident,

The FAA has adopted a rule establishing that all aircraft flying below 10,000ft m.s.l will be limited to a maximum speed of 250kts effective December 15, 1967... (to) provide a more realistic 'see and avoid' environment in the airspace below 10,000 feet mean sea level (MSL) where the traffic congestion is greatest ....

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    $\begingroup$ This is the correct answer, but lower speeds below 10,000 are also advisable for other reasons, such as that being where pesky birds like to hang out and bird encounters at 400 knots can be quite unpleasant. $\endgroup$ – reirab Sep 10 '15 at 20:28
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    $\begingroup$ There is also a lower limit of 200 Kts in VFR corridors through Class B airspace. $\endgroup$ – Dave Sep 11 '15 at 14:19
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    $\begingroup$ Nice. I would be interested though in any details regarding European airspaces. Did they just use the knowledge from that accident in USA and followed FAA's example? Or they performed their own research? $\endgroup$ – Stelios Adamantidis Sep 11 '15 at 16:12

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