What is the common denominator for cities with high mode-share for bicycles?
Latitude plays a part (US cities that have a significantly better than average share of trips taken by bicycle are mostly along the 45th parallel), as does having the correct demographic (usually anchored by a major public university or three in the urban core). Good public transportation is also usually present. But these factors are not always present simultaneously.
But one visible clue ALWAYS proceeds a high bicycle mode-share: pedestrians. Cities with high numbers of pedestrians are the cities with high numbers of bicyclists... they go hand in hand. High population density is a required ingredient for both modes to have significant numbers, but it's always the presence of a large number of pedestrians, making short trips, that indicates a city is likely to become a place where bicycles can become a major part of the transportation system.
The limit to a comfortable pedestrian trip is roughly 1/4 mile. 1/2 mile and longer trips are hikes, and the number of trips drops dramatically. Where do peds walk 1/4 mile? From home to work, in many cases, and from home to get daily groceries, or a meal, to school, or to pick up laundry. or from home to transit/transit. That's why building more sidewalks won't dramatically increase pedestrian activity in low-density neighborhoods... the trip origin/destination points are too far apart.
A cyclist's ideal casual trip is 4X that of a ped, or roughly one mile. Beyond one mile, and a bike trip is a bike ride, which alters the mindset. Minus the transit component, the rules for ped trips are about the same for cyclists.
If you know of an exception, please let me know.