The reference is in relation to culverts/portals for road projects. The suggestion provided by tihomir is not correct. The text relating to the transition from roof ceiling to roof, refers to the haunch (voute). The transition slab is not related to this.

A transition slab is generally a reinforced concrete slab used where a road 'transitions' from being constructed on top of a concrete structure to being constructed on fill to mitigate any negative effects from settlement in the fill. This is an SVV requirement where fill exceeds 3m.