ok. It's not really a "crippling condition" when natural experiments do crop up from time to time, though.
For example: if Amsterdam suddenly had an MHL, that would be one. In fact, any city that instituted MHL would be a natural experiment. The number of riders, the number of accidents, the % of accidents that involve injuries -- all those things would change, while the city design and transit situation (other than bikes) presumably would not.
For example: if Amsterdam suddenly had an MHL, that would be one. In fact, any city that instituted MHL would be a natural experiment. The number of riders, the number of accidents, the % of accidents that involve injuries -- all those things would change, while the city design and transit situation (other than bikes) presumably would not.