While I appreciate they are trying to promote cycling, I am not sure this was the best of ideas. Cars and bikes should be able to "co-exist" pretty well in cities, assuming certain provisions of infrastructure and cyclists playing fair by not acting like idiots (many do in my experience). Shutting cars out just creates a nuisance for people who need cars for work or their disability. It doesn't prove anything. Its just rather token.
I think even token is probably an overstatement. Roads are closed for various events all the time, this is just another one. It really has very little to do with "promoting cycling" or anything else. Over here they shut down half of Surfers Paradise for three days every year for a car race, and very few people bat an eyelid (or at least nobody's listening if they do complain). If people were really trying to promote cycling, there are other ways they could go about it. I think this is more of a "day out" than anything else.
Incidentally, I recently had the pleasure of riding on London for the first time, and I have to say I found it an extremely pleasant experience. Just to ride around through the sites, the architecture and the well-behaved drivers (relatively speaking) was thoroughly enjoyable.
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I am clinically insane. I am proud of it.
That is all.