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Classes of crossings==Crossings can be classified according to the number of single and split(double) roads involved in the crossing.
Simple road crossing a split road (SSD)
Split road crossing a simple road (DDS)
Split roads crossing (DDD)
Ah, the "Mother of all Crossings"! We are lucky that almost all crossings look like this. It is possible to define each and every turnrestriction with a single node, and U-turns do not happen when all 4 roads in this junction are locked (No, there is no evidence for
this. But each and everytime a simple crossing does allow a U-turn, at least one of the roads was found not to be locked).
Split road crossing 3 simple roads (DSSS)
This is an ugly crossing if you come from/go to the SE. You get a direction to "Turn left, turn right". First check if the double road can be made into a single one, so you get a SSSS. If that fails you may want to join the double lanes into a single node, creating a sort of half-mapcat-bowtie.
Split roads crossing 2 simple roads (DDSS)
This one allows for U-turns to be made if you enter over one of the double lanes.
Split roads crossing 1 simple road (DDDS)
Ugly! If you can not turn this one into a SSSS or DDSS, consider adding an extra fork on the single road and create a DDDD.
Split roads crossing (DDDD)
This crossing allows U-turns in each and every direction if no precautions are taken.