The scores are from 0-100 and “based on the availability of bike infrastructure (lanes and trails), the hilliness of the area, destinations and road connectivity, and the number of bike commuters.”
Bike Score is new, though, and hasn’t been implemented everywhere. They are expanding, so if your city isn’t currently listed, let them know you want it added.
Perhaps in a fitting show of how our cities and neighborhoods are not designed to biking, number one ranked Minneapolis doesn’t even get a B for its effort. The 79 score it has is the highest in the country.