First of all, I've never used polaroid transfers. I don't know what you can stick polaroid transfers too, or how they hold up to sunlight or rain. So this idea may be doomed from the start, but...
When I drive around, I regularly see traffic accidents. My first thought would be to make a stencil of an accident, and put the stencil up every where I've seen accidents. But then I thought, as visual proof for viewers, a polariod transfer of the accident put up where the accident might be better in a way.
The goal? To have a the stencils/transfers at the location of past accidents to remind people to be safe. And the more stencils in an area, hopefully people would be reminded to be extra aware. I got the idea from Japan where on streets with lots of accidents, they have a running tally posted at those intersections.
And who knows, if the idea takes off, the city might take over in a way. While it would never admit grafitti was doing good, they might start putting up warning signs to warn you that you are nearing a dangerous area.