We use a third party to calculate routes between points on the map. When you drop waypoints on the map, the locations of those points are sent to the routing service. The routing service calculates a path between those points that follows known roads and returns it. I then plot the path as a line on the map. 

Unfortunately, with the increased activity on the site, we've busted through the monthly cap on route requests so suddenly, 48 hours ago, creating routes on maps no longer worked. Bummer.

I've found another routing service that provides many more monthly route requests. I've implemented support for them so laying out routes on maps works again. As a bonus, while they limit routes to 25 waypoints, I implemented support for stitching together multiple responses so we can now have routes with many waypoints.

The routing algorithm they use is different than the first provider so you may notice some differences. If you're building maps here and run into problems or notice anything weird, please, as always, let me know. 

For an overview of routes versus tracks and some background on all this stuff check out this article: