Algorithm

The RouteXL planning algorithm uses travel times and a optimization method to minimize total travel time.

Travel times

To find the best route, the travel times between all locations are required. While most other route optimization tools use geographic distances (as the crow flies), RouteXL uses a crowdsourced road network. OpenStreetMap is the free Wiki World Map, an openly licensed map of the world being created by volunteers using local knowledge, GPS tracks and donated sources.

Learn more: https://www.openstreetmap.org/

Optimization

Routing multiple addresses is quite a puzzle. With 20 destinations the from-to travel times matrix has roughly 20 x 20 = 400 elements. The number of possible routes is even bigger, approximately 20 x 19 x 18 x ... x 3 x 2 x 1 = 2,432,902,008,176,640,000.

Mathematicians call it a "hard" problem and there is no final one-size-fits-all solution available. They even have a name for it: The Travelling Salesman Problem. Indeed, humans can fly to the moon, but can't solve this problem.

Learn more: https://en.wikipedia.org/wiki/Travelling_salesman_problem