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Why prevent U-turns?
* We do not want Waze to suggest illegal
routings. Ever. U-turns are rarely ever allowed, but Waze will suggest one on each and every occasion where it thinks it is appropriate. ( But in Germany U-turns are allowed in most cases, except they are permitted by sign.)
* Ad-hoc routing
gets better if no U-turns are possible. If you have to divert from the route you are navigating on, and leave the purple line, Waze will calculate a new route for you. If it is possible to make a U-turn chances are that Waze will send you through that U-turn back to where you came from - the purple line was the best route, after all. But - you may have diverted from the purple line with on purpose - to avoid a congestion that loomed in the next curve, or a road that is closed for repairs. If opportunities for a U-turn are rare it is more likely to happen that Waze will route you around the next block or two so that you get back on track AFTER the congestion.
*'''U-turns can be prevented by reducing the length of the segment crossing the median to less than 15m and leaving this segment unnamed.'''
Not much to say about this
one - except that it is the ideal 3-way crossing: simple, stable, solid. Not much can go wrong with this one! If you have to deal with any of the following types of crossing, you may first look if it is possible to turn them into this type, since it will save you from a lot of trouble later on.
The next simple type of crossing
already has limitations. The way it is drawn here there is nothing that stops Waze from plotting a U-turn. Coming over the double road it takes two left-turns to get back on the road you came from. Typically that is NOT what you want.
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 that. But each and everytime a simple crossing does allow a U-turn, at least one of the roads was found not to be locked. If all roads are locked, Waze routes you around a block instead of making a U-turn).
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.
*2 Dual roads crossing 2 single roads (DDSS)
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.
*4 Dual roads crossing (DDDD)