A plain Hi viz jacket is far more effective. The sight of any hi viz jacket triggers an almost pavlovian reaction in many people , officialdom! As soon as they read "polite " they realise that they have been ,at some subconscious level, tricked . The result is anger. on the other hand ,an unmarked jacket leaves that element of doubt. Do the police always wear jackets with "police" written on it? Do I really want to find out? Not really!
I observed the "unmarked jacket " effect recently when I wore my new (not covered in horse poo or engine oil) Hi viz jacket whilst flying from Luton airport to Cluj Napoca . It was that sunday when we had all the snow and all the flights were cancelled . I had a never ending stream of bewildered travellers asking me what was going on , confused police ,border control ,even restaurant staff all thought I was official in some context or other . This is the magic of the unmarked Hi viz Jacket.