Abstract for: Bicycle Helmet Laws, Safety-In-Numbers, and Bikeshares

The safety of cyclists is strongly correlated with the number of cyclists. While a causal direction for this safety-in-numbers effect has not been proven, there is reason to expect causality in both directions on various timescales. Well-implemented bikeshares lower barriers to entry and increase convenience, facilitating many new trips by bicycle. Helmet laws protect some cyclists during collisions, but reduce the number of trips by bicycle, especially impacting bikeshare systems. If helmet requirements reduce bicycle use, the immediate increase in risk is likely to decrease bicycle modeshare, further increasing risk. The hypothesis is that this short-term effect activates several longer-term reinforcing feedbacks. This paper sketches some likely causal loops connecting bicycling safety-in-numbers, bikeshares, and helmet laws, to life expectancy. The results presented here are not intended to be predictive, but rather to demonstrate a potentially dangerous system behaviour, and to motivate more detailed study.