Pinyin in practice

I've been starting to learn Mandarin for about 3 years (still not got very far) But this had me fascinated because learning to speak and listen to Mandarin is a different ball game to learning the ideographs traditionally associated with Chinese languages. So, what happens when a quite different way of writing starts to be commonly used, quite the variant on diglossia; digraphia. See here: Language Log � Pinyin in practice:

many Chinese police cars and uniforms have written on them GONGAN ("public security") rather than "police", and sometimes not even 公安.
When I encounter such situations, I often wonder:
1. why they choose to use pinyin and NOT Chinese characters
2. why they choose to use Mandarin in pinyin instead of English
3. for whom the sign is intended

One of the comments says this in response, which seems possible to me.

1. pinyin is unambiguously Mandarin the national language and not other Chinese languages
2. pinyin is China's legible 'face' to the world, as a lot more non-Chinese can read and ecognize 'Beijing' than would recognize the characters.

I'll also develop Carl's idea. If it's true that school children learn pinyin before*characters then the argument might be made that more Chinese have a fuller awareness of pinyin than they do of characters. That is anyone who's spent a year or two at school and can more or less speak Mandarin can at least decypher pinyin while only those with a lot more schooling can decypher characters.

Those who have becomevery literate in characters might find it harder to read Mandarin in pinyin but average folks on the street probably find it easier, especially in short messages as in public signage.

It's the interplay of meaning-making, power, economics, perceptions ....