A common version of these indicators (the simplest contour-based indicators) also makes use of thresholds (e.g. number of services reachable in 30 minutes), with all activities within and beyond the threshold treated as equally important. These indices include an impedance function or sensitivity cost in the equation, which decreases the desirability of routes according to given characteristics - distance or road quality, for example. Simply put, destinations that are closer to the origin, or served by better roads, are given a larger weight than those that are farther or have poorer roads.