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Why does the taxiway centerline extend on this long distance on the runway?

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    $\begingroup$ It’s not leading to the runway. It’s leading off the runway. It indicates where nose of the plane should be in order turn to get off the runway and not hit anything on the way off. You will also see the yellow lines crossing the runway where a taxiway crosses a runway. $\endgroup$
    – JScarry
    Apr 13 at 19:41

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On a long runway (see below), for a taxiway used as an exit, the lead-off starts 200ft before the tangency point. This corresponds to the length of a stripe (120ft) and a gap (80ft).

This lead-on is important for low-visibility operations to make the pilot aware of the proximity of the curved segment. A random taxiway lead-off at KSFO:

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(This place on Google Maps)

ICAO recommends this distance in Annex 14 of the Chicago Convention: 30m (100ft) or 60m (200ft):

5.2.8.7 Recommendation.— At an intersection of a taxiway with a runway where the taxiway serves as an exit [...] The taxiway centre line marking should be extended parallel to the runway centre line marking for a distance of at least 60 m beyond the point of tangency where the code number is 3 or 4, and for a distance of at least 30 m where the code number is 1 or 2.

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(Code 1 runways are less than 800 meters long, Code 2 runways are 800-1199 meters long, Code 3 runways are 1200-1799 meters long and Code 4 runways are 1800 meters or more in length.)

This ICAO recommendation is usually implemented by State members, for the US/FAA in FAA Standards for Airport Markings, AC 150/5340-1 at §4.2.3.2.1. E.g. for the case of a taxiway joining the runway at an intermediary location:

For taxiways that intersect a runway at any other locations than at the runway end, as shown in Figure A-16, the taxiway centerline marking curves onto the runway and extends parallel to the runway centerline marking for a distance of 200 feet (61 m) beyond the point of tangency with the runway centerline [...]

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