What is the definition of a high speed turnoff?
In the old days we called them Rapid Exit Taxiway (RET).
"Rapid Exit Taxiway — A taxiway connected to a
runway at an acute angle and designed to allow
landing aeroplanes to turn off at higher speeds
than are achieved on other exit taxi-ways and
thereby minimizing runway occupancy times."
Page 21
http://ww1.jeppesen.com/documents/av...ry-legends.pdf
See
http://www.gcmap.com/diagrams/pdf/EGLL.pdf
"HOLD N4W/RAPID EXIT TAXIWAY (RET) REDESIGNATED AS UNDER CONSTRUCTION." as an example of what one looks like.
See page 166
http://ww1.jeppesen.com/documents/av...port-signs.pdf
Lots of information on radius of turn / maximum speed etc here :
icao_doc_9157_aerodrome design manual-part2.pdf