When making 90 degree turns with a radius of 14 or 30 tiles, a pair of quarter turn segments will be combined into one half turn segment, but an equivalent R22 turn will remain 4 separate quarter turn segments. R22 half turn segments exist, but are only used when specific tiles are blocked (2nd image, the 2 molten channel ghosts on the inside edge of the curve).
The R22 Half turn segment is also slightly more expensive than a pair of R22 quarter turns.
