cost_to_closest         Minimum travel cost to closest N number of
                        opportunities
cumulative_cutoff       Cumulative access based on a travel cost cutoff
cumulative_interval     Cumulative access based on maximum travel time
                        interval
decay_binary            Binary (a.k.a. step) decay function
decay_exponential       Negative exponential decay function
decay_linear            Linear decay function
decay_power             Inverse power decay function
decay_stepped           Stepped decay function
floating_catchment_area
                        Floating catchment area accessibility
gravity                 Gravity-based accessibility measures
