When looking to measure the success of any project, performance metrics help determine the efficacy of what was done. Measuring any project over time using these indicators shows both progress and opportunities to take iterative course corrections. Often, quantitative measurements are used over qualitative ones, primarily because the former is more objective. With transportation, the easiest metric to track is the number of miles of something. For instance: the number of miles driven, or the number of bike lanes installed, the number of miles of roadway.
These do a great job of enabling an efficient use of resources (an amount of thermoplastic to order for striping the bike lanes or knowing the total amount of asphalt to anticipate for road repairs). When you look at the effectiveness of a project, such as “how can we get more people to use the bike network”, just offering more miles is a poor metric to use for influencing strategy and determining results. Yes, on the surface more miles of bike lanes are good for a bike network, but they don’t in-and-of themselves influence use. Connectedness and accessibility do, however, and one of the best ways to measure those is through percentage of coverage.