The fact that so many people choose to cycle in Japan has a lot to do with the fact that they are not forced onto the road with motorized traffic. Young children, teenagers, women with kids, middle-aged women, the elderly, the unfit and unsporty -- all are well represented in the utility cycling demographic in Japan. I know because I cycled in and around Nagoya, Yokkaichi and Kyoto for 10 years.
Where I'm living now, in Christchurch, New Zealand (mythically the "most cycle-friendly city"), less than 2 percent of traffic consists of cyclists. Yet 12 cyclists were killed on the roads here last year. Cycling on sidewalks is illegal (and enforced). As a result, the above-mentioned demographic groups are poorly represented among cyclists. They won't ride on the road with traffic -- even where there are cycle lanes.
The vast majority of cyclists in New Zealand are young men, assertive advocates of "vehicular cyclism," riding mountain bikes and road racers as fast as they can, who for the most part seem to think that anyone on a bicycle should behave as they do. The cycling advocacy movement is strongly influenced by their perspective, perceptions and assertions -- all backed up with statistics that play into the hands of the auto lobby. Keeping middle-aged women, children and the elderly off the pavements keeps them from taking up cycling at all.
Local government councilors, who have the last word, are hamstrung. Without the political support that a universally inclusive cycling demographic would give them, they cannot secure funding for the establishment of a cycling infrastructure that would have actual separation from motor vehicles.
If the Japanese police were to force all cyclists onto the road, I have little doubt that you'd see a death rate from cycling similar to what we've experienced here in New Zealand.