Hi-Viz is only useful if the driver(s) are actually looking and paying attention. There's plenty of examples of drivers hitting static objects that are festooned with hi-viz colouring and reflectives.

Bicycle helmet safety standards are generally for speeds up to 12mph and/or a static drop from 2m onto a hard surface. They are specifically not designed to withstand the forces/accelerations involved in a multi-vehicle collision.

(One of my bug-bears is when people point to a destroyed bicycle helmet and claim that it saved their life. Helmets are designed to compress and are particularly weak when subjected to tensile/shear forces. If a helmet splits in two, then it most likely wasn't being effective)