Coming from a Chicago Critical Mass perspective....
Bear in mind I havent gone to one since late last year, but they can be alot of fun in general, and for the most part. In the beginning, people are really excited, and gather at Daley Center. Friends meet up with friends, or see friends they havent seen because EVERYONE is out on their bikes for this. These rides literally stop traffic for probably a good 10-20 minutes wherever they go through. The energy level is really high, most people are in good spirits because its the weekend. Some people in the pack have great stereo systems on their bikes, making that fun to be around.
NOW, it's really interesting because as the bike ride starts, many people have brought alcohol in some form or another and it slowly starts being consumed especially when you're out of downtown and the loop. So many types of people are out, amongst bikers themselves, riding a crazy list of bikes from tall bikes, to rat bikes, to fancy bikes, cheap bikes, you name it they are riding it. I saw once a bike that was covered in a faux wood tape so his whole bike looked like it was made out of wood (sorta

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As the ride carries on, many people begin to show thier hatred towards cars. Many probably are complacent about it and are just out for the ride but very many are aggressive towards cars. Being stuck in traffic is a car drivers nightmare, and to be sort heckled at by a bunch of bikers, they become more aggitated. They see bikers venting at them, some weilding beers in their hands like its cool to be a biker and lame to be in a "coffin". Riders can run their mouths at drivers and many of them cops in their cars policing the ride as we go through. It really can be a very climactic time and will cause some serious reactions. But drivers can certainly egg bikers on just as much so it's a double edged sword for sure.
The last critical mass I went to, a driver got so frustrated for being stuck at an intersection, with bikes in front of their car to ensure they couldnt get through, finally pushed their way through forcing bikers and their bikes out of the way. People began to surround the car and many heated words were exchanged, resulting in two windows being smashed by bike locks. They didnt let this car move until all of the bikes had passed. Another seperate incident, a young boy got hit from a car not being stopped came through and ran him over. LUCKILY, he survived with little injury and needed a new bike.
The problem I have with critical mass sometimes is that I know that we are their to stand for not relying upon a car for transportation, but alot of time it ends up being kinda of one up-ing eachother situation. Bikers think they are better, cars think they are better and its this huge clash amongst people not really accomplishing things. It really bothers me when I see alot of bicyclists drinking alcohol, being kinda roudy, being flagrant and flippant when it's really their motives to convince people that bikes are traffic and just as impactful so please stay aware of them. I'm fine with people acting like that, alot of bikers are and thats alot of the fun, BUT when you are in public eye and alot of people are being inappropriate, we as a whole can get lumped into their opinion. No, I shouldnt care all that much, I am confident biker, but I know that how are judged can effect other people so being aware of that is key. I just think it sends the wrong message sometimes.
So, I am not surprised that a cop got so infuriated with a biker enough to hit him. Is it acceptable? Well, no, but people just need to keep themselves in check and just enjoy the dang ride!
Anyways, enough ranting. They are just my opinions and don't adhere them to anyone else.