For most cities, biking and walking infrastructure is the lowest risk, highest returning transportation investment that can be made.
A bike is the cheapest and easiest way to expand the range an individual can easily travel. A bike is a near-universal technology that is time-tested and difficult to improve upon.
Where traffic speeds exceed a safe neighborhood speed (greater than 20 mph), separated infrastructure must be provided for biking to be safe. Where traffic speeds are low (less than 20 mph), it is possible, using great care, to build safe biking infrastructure where people ride in the flow of traffic.
More than helmets or any other safety device, developing a culture of walking and biking is the most important safety improvement a community can make. The safest thing for people who bike is to have more people who bike.
Rather than large projects intended to transform the community, the best investments for building a culture of biking and walking are incremental. Start with people who are already biking and find ways to make their routes easier to travel. The immediate goal is to make it easier for the next marginal person sidelined by safety or comfort concerns to start biking.