The ICAO rapid exit taxiway design speed is 50 knots (93 kph) for Code 3 and 4 aircraft for exit speeds under wet conditions.
In setting this speed, there is something of a safety margin built in, and the assumption is that the aircraft is usually travelling slower when it exits – the nominal turnoff speed is 30 knots for Code 3 and 4 (Aerodrome design manual Part 2 – section 1.3.12).
It makes sense that the operating manuals are targeting something like the 30 knot mark, because that leaves all the safety margins in place for the unusual operation.
The design of the rapid exit taxiway is rather more than just a curve radius for maximum speed. It also provides for a straight distance after the turn-off curve sufficient for an exiting aircraft to come to a full stop clear of any intersecting taxiways. It is based on modest lateral accelerations, and very gentle deceleration rates for the curved section and slightly more (but still gentle) for the straight sections.
I can't recall any accidents as a direct consequence of an aircraft using a rapid exit taxiway (although that statement is so bold as to beg correction. Update: searched the NTSB accident database from 1990 to 2008 and found none), so ICAO has probably got the numbers reasonably right.
Last edited by OverRun; 28th December 2008 at 08:00.
Reason: NTSB accident database search