R⁵ is a fast routing engine for multimodal (transit/bike/walk/car) networks. It is currently designed for analytic applications but there are plans to eventually have it support point-to-point journey planning. The routing is "realistic" because it works by planning many trips over a time window, which is more reflective of how people use the transportation system than planning a single trip at an exact departure time---very few people leave for work at precisely 7:53 AM every morning, and even fewer leave at a precise time when going to a non-work event. There's more information on our thinking on this particular point here.

R⁵ grew out of several open-source projects. The focus on analytic applications, and the core development team behind R⁵, came from OpenTripPlanner. Many of the algorithms, as well as the name, came from r4.