Open Risers and Overhanging Treads on Stairways
Open risers, gaps between one tread and the next, cause real difficulty for people with a vision impairment, especially with a light source behind the stair: the strobing effect between treads can trigger vertigo while ascending. A walking stick can slip through the gap, and an overhanging tread creates a lip that catches a foot or a prosthesis, particularly on the way up.
The BCA covers this at D3.3(c), calling up AS 1428.1 for the construction detail, including the requirement for opaque risers.
Getting it right
Every riser needs to be enclosed and opaque, with no overhanging lip. It's a simple requirement, and one of the cheapest to get right at design stage compared with retrofitting later.