I had a similar situation happen. I was running on the sidewalk one day and saw a cop come at around 30-40mph down a merging street. As he approached the intersection I could see we were going to meet. I saw that he had a stop sign, so I continued thinking he would stop. As he approached the stop sign he barely tapped the breaks and cut me off going 25mph, never seeing me. I yelled but he didn't hear/ignored me. I saw his license plate, memorized it and promptly went down to the sheriffs office and explained the full situation to an on-site officer. Later I got a call from the chief of police applauding me for what I did and offered to help me with a formal complaint. I declined and allowed him to deal with it internally (an ass-chewing was the likely punishment)
The cops in my town can be dicks and hound the streets for tickets at the end of every month. The ignorance drivers/cops show goes unchecked and think that a treadmill is an answer to their problems. If people want us off the streets then they should all take public transport everywhere so they too can contribute to less congestion.