Hi @karlwoll15 and welcome to the forum!
Here’s a lengthy answer with a bunch of links for your enjoyment. Happy Striding!
Your questions has two parts:
1. How is the percentage calculated?
The site’s creater answers that in this thread, where someone asked the same thing. Here’s the formula:
A few other links that may be relevant reading:
- Another thread discussing the same question.
- Alternative Completness Metric: having additional or alternative metrics occured to me a few months ago as I was thinking about this very subject
2. What counts as a street?
Actually a good question… and one that has been discussed at length here. Short answer is that it is just streets: not alleys, not pathways or trails, and not major highways
Here are some other links that may help answer the question further
- Adding Hiking Trails: a thread to discuss whether trails should be added in
- Greenways and Parks: same as above, but with… you guessed it… greenways and parks
- 100% Strategies: pretty interesting strategic/philosophical discussion about how people get those hard-to-get streets
- All the Streets in Burnaby: another interesting discussion around some people doing everything or not
Also if you’re interested, there is a wiki post with more information about searching the forum you can check out to help navigate the site.