There's a lot of interesting stuff going on in there. First, for the curves itself, it does a kind of naive approximation optimisation and builds a curve out of

There's a lot of interesting stuff going on in there. First, for the curves itself, it does a kind of naive approximation optimisation and builds a curve out of line and arc segments that tries to follow the control points as closely as possible. Then it does an intersection step between all roads and procedurally generates correct connections between all incoming and outgoing lanes of each intersection, including (somewhat) reasonable traffic light timings for each "turn group"