There are two separate issues here, school uniform and road safety. The issue raised is, however, one which your governing body might well wish to consider.
Strictly speaking, of course, the safety of pupils after they have left the premises is not the school's responsibility, but education in road safety is a universal concern.
The problem is not likely to be resolved by your governors requiring all your pupils to wear bright yellow coats. The pupils would probably refuse to wear them.
On the other hand, if you had a serious campaign to persuade them to wear a luminous belt or patches on their bags or sleeves, you might achieve your goal.