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The combination of clever idea with simple execution makes this a great concept with a high likelihood of success. Self-interest is the best indicator of behavior and this concept addresses that at every angle. The advertisers get green credentials, the individual gets a better way to get around in a crowded city (and eventually notices the health benefits) and we all get less traffic and a better environment. I don't think there will be any problem in people valuing their 'free' bikes. Why would you abandon something that gives you another transportation option and in some cases (heavy traffic in the center of London) a better and faster one? Plus, the mobile mechanic stations mentioned in the comments will not only serve to prevent abandonment but will also tighten the link between the advertisers and the users (I suspect you wouldn't be able to get your bike fixed if you had torn off the advertising), thus making this an even more valuable proposition from both sides of the equation. My suggestion is to give away helmets too: promotes bike safety and gives your sponsors another space to advertise.