With the implementation in #3298, large circular junctions and roundabouts are now treated uniquely from the navigation perspective, the user is given the exit count. There is currently no lane recommendation before such roundabouts.
Such large circles often have a sophisticated lane layout, consisting of a sequence of different
turn:lanes patterns. Some are designed in a ‘spiralling out’ fashion, which has the effect, that queueing in a particular lane at the entrance leads you to a specific exit without changing lanes, or only changing to a new innermost lane springing off from the middle.
Algorithmically determining the recommended lane for a particular exit might be difficult, since current tagging of turn:lanes insufficiently covers the lane transitions. While there is a proposal for
placement=*, it is not well adopted since it is both complicated for the mapper and the data consumer.
Thus my idea is to have a specific lanes tag at the entrance roads, e.g.
circleexit:lanes=n|n;n|n. So when the leftmost lane serves exit 5 and 6, the next exit 4 and 5, the next exit 2 and 3, and the rightmost exit 1, you would map
circleexit:lanes=5;6|4;5|2;3|1. So, once the router has determined to take exit 2, it is easy to get the appropriate lane(s) green. The numbers in the tag should be relative to the entrance, so the exit number directly matches the Nth exit announced by the router.
This tagging should be in addition to the
turn:lanes which represent the arrows painted on the ground.
My plan would be to discuss here first for feasability, then announce on the tagging list and have a test implementation. Or test first and announce then. I’d be happy to cooperate with the Berlin circles.