traffic police

The text is coming from a police notebook dated from 1973. Regarding the traffic police.

I have 2 questions



1.)Why is the traffic police listed as BD?(Biet Doan) or does BD in this case have another meaning?, as i cant imagine that Biet Doan does fit the description task to the traffic police.

2.) in most English speaking police manuals, dated from 1968 to 1971, originally translated from Vietnamese, the traffic and order police are listed in the same chapter, or it seem listed as one unit. But each having different task, the problem I do not have the original Vietnamese text as to differentiate the exact Vietnamese unit name, the traffic police is lưu thông, and the reference to the order police as far i guess should be An Ninh - Trật Tự.
But both do wear different patches, so why one unit with 2 different type of patches.? have they been merged at a certain time?

HCMT