At a Lincolnshire airbase, FSW Orders mandate the wearing of hi-vis vests on the dispearsal, and I understand that some groundcrew have fallen foul of the rule. The aircrew "should" wear them, but many refuse because they think that they are above all that (or too idle to put theirs on). The only chap to have been hit by a crew bus while walking on the dispearsal was infact wearing hi-vis and this is used as a reason not to wear it. However, I consider that it is like not wearing a seat belt in the days before compulsion - if you get hit and you were NOT wearing one then I expect that any compensation due would be markedly reduced. Indeed, I expect that the driver would sue you for the distress that your negligence had caused him!
Bottom line - it costs nothing to wear one, so why not - if you get hit you will get more money! Its like the thread on whether to wear gloves or not.