Every community is different, which is why further expansion of the federal role in infrastructure spending is not a good idea. Let the locals figure out what is needed. And make the locals pay for it, which will cut out much of the nonsense. That said, in many major metro areas, shoehorning more cars, more parking, and more arterial roads into already heavily congested areas is a fools' game. On my aggressive days, I think we should dispense with eminent domain and let motorists pay full market value if they want to take other people's sidewalks, tree plats, and front yards for another traffic lane, and sacrifice urban parks for yet another access ramp. That would stop sprawl pretty quickly, and help induce people to live closer to their jobs.
Bike paths? I'm generally in favor of building bike (and pedestrian) routes into the mix from the start, or when major renovation or replacement projects arise. Retrofitting can be costly, but when accommodations are made up front, the cost is much less. The form these should take will vary. Off-road trails utilizing existing parkland and stream and rail corridors are great, where the opportunity exists. These, however, will constitute a small fraction of the mileage needed for a coherent network. On quiet residential streets, bikes and cars can share safely, with marked bike lanes thrown in where appropriate. On busier streets, dedicated bike lanes or wide sidewalks are solutions. It is best, of course, to route bikes entirely away from busier streets, but that only works if the side streets are accessible and bikeable. If you see cyclists dodging heavy traffic, it's only because the transportation planners have left no other way for bikes (or pedestrians, for that matter) to get from A to B. Unfortunately, this often happens.
A recent example: two weeks ago while exploring a new route I rode a seven mile detour because a railroad across my route was crossed only by two parallel arterial roads, one an interstate, neither of which had a sidewalk. That's seven miles to move about 100 years ahead, where side streets opened up again. That's lousy design, in an area with a major park on a river, where there are pedestrians, dog walkers, and joggers as well as bicyclists, and you can't move 100 yards from A to B without a car. That's the kind of thing that makes us irritable.
The motorists, of course, never stop to reflect that people actually live there and might like to get around, or that there were certainly once sidewalks that allowed them to do so, before those were sacrificed to the car lobby.
The point is not simply to make the world safe for recreational cycling. Walkable, bikeable neighborhoods add value. They are attractive to young adults with an appetite for active lifestyles, they're attractive to families with kids, they're great for retirees, and they might even entice a few people out of their cars for a little exercise. No, not everyone will take up biking, but it's an activity that most people find pleasurable, and many more people would do it if they had good places to ride. I've come to think of bike friendliness as one of the easiest community quality of life enhancers available -- and again, it doesn't have to be expensive if it's built in from the start, or in the course of major renovations. (Put a pedestrian/bike path -- a wide sidewalk -- on the #"&!(%## bridge, already, and do it everytime a bridge is replaced.) Think of it as an incentive to gentrification, which it is; 20-something yuppies love jogging and biking trails, and it's better for them to help reclaim older, close-in neighborhoods than to live in the far suburbs and add to the congestion on the highways.
In my area, the biggest problems seem to exist where sidewalks (and in the suburbs, wide shoulders) were sacrificed years ago to create more traffic lanes. Roads that easily and safely accommodated pedestrians and bicyclists when they were built have been turned into commuter sewers, and rendered unfit for anyone else. They become barriers to anyone not in a car. Motorists need to understand that this is a serious degradation of the neighborhoods they traverse, and that compensatory spending on bike and pedestrian access, and landscaping to buffer the sights, sounds, and smells of heavy traffic, are appropriate uses of transportation funding. It's another case of internalizing the externalities.
Here is a rule of thumb. Forget bikes for a moment. Pedestrians should be able to walk safely along ANY road in an urban area. After all, pedestrians actually live in the neighborhoods that motorists consider drive-through country, and they should be able to get around. And pedestrians should be able to conveniently cross roads without long detours, which means more stoplights or pedestrian over/underpasses on commuter arteries. Make those sidewalks wide enough to pass, and you've solved the bike problem as well.
Don’t get me started on bike paths. The lefty Mayor of Pittsburgh has been irking people by turning perfectly good roadways and bridges into bike paths. The city core is being taken over by Millenials, gays and climate change bedwetters, which forms the base of his coalition.