Perhaps the stupidest public policy idea I have ever heard of is the proposed bike tax. It is not worth talking about the proposal itself as it is not going anywhere and is, as I think I mentioned, stupid. But what is interesting to me is that, in fact, the appropriate public policy is to subsidize bikes, not tax them.
Why? The negative externalities associated with bike riding: virtually none. Minimal wear and tear on roads, sometimes a slight slowdown in traffic and a extra line of paint for a bike lane. Positive externalities associated with bike riding: lots. Reduced congestion and emissions from those that bike in lieu of taking a car, and better health and fitness of riders reducing the toll on the public health system. Public economic teaches us that to get an efficient amount of economic activity that has externalities you have to get the price to reflect the true cost of the activity. In this case the true cost is less than the price of a bike.
Okay, so this will never happen, but we do get part of the way by providing a better biking infrastructure.