only allow the leftmost source lane to turn to any destination lane. As
a future improvment, need to handle multiple explicitly tagged left turn
lanes, but this gets closer to reality, particularly helping some crazy
maneuvers along Mercer in downtown.
*or right
Also had to update lanes along Madison and fiddle a bit to keep
lakeslice running. Spotted some major traffic signal bottlenecks due to
stage generation falling back, will iterate on that separately.
raw string" stuff from the previous commit.
Add tests of a few interesting intersections. The results right now
aren't ideal, but this sets things up for fast iteraton.
