This graph compares the scheduled average speed by route with the average number of bus stops per mile for all bus routes in the WMATA published data.
The overall trend is for buses with fewer stops per mile to have higher average travel speeds. Buses that use grade-separated routes, like the 5A and other freeway buses, tend to have both fewer stops per mile and higher travel speeds, while buses in dense urban areas, like the 90s, tend to have more bus stops and travel slower.
One significant outlier is the 6 stops per mile, 20 miles per hour point you can see on the graph. That route is the J13, which only travels once per week at 6am on Saturdays, and flies through its route with relatively low congestion and ridership.
I’ll be working on making graphs comparing WMATA to other transit agencies that publish data, as well as showing the different data for peak/non-peak and weekday vs. weekend.