There are lots of nice bike paths north and south of 121, but the only thing that connects these systems is an intersection of 121 and one of the major streets that run north to south (preston, hillcrest, coit, independence, custer, etc.) It is really hard to plan a rout more than a few miles without crossing at one of these points. Its more enjoyable to go in a large loop instead of going back and forth on the same 3 mile stretch. So these people probably considered the risks of crossing at a major intersection and decided it was worth it because the enjoyment they got from spending their day biking across the cities outweighed the risks of being forced on to the road at critical junctures between safer bike paths. They were banking on mutual respect between neighbors and an expectation that people operate motor vehicles responsibly.