Being born and raised in NYC, I can tell you a little history about the "NYPD Traffic Enforcement" unit.
For many years they were refered to as "brownies" - the cars and uniforms were brown with white lettering.
For many many years, these people never got respect from motorists. Many times they were assaulted, intimidated and threatened.
One day, NYC decided to grant them a somewhat higer status in terms of protection and began painting the cars in a dark blue. they also appended the word "NYPD" to make their title seem more offical and to give motorists an authoritarian and powerful or police-like impression of them.
I hate to say it but these are some of the laziest city workers around. They are usually uneducated, slothy and not too bright. Illegally parked city vehicles from the various city agencies are an incredibly common thing here.
There have been many reports of these abuses by city vehicles and the city has made it clear that it is illegal for these individuals to do this, but nothing really happens.