I've received and tested new information from Waze which radically change what we thought about how U-turns are prevented.

I'll be working to elegantly add this to the wiki. But in all preliminary tests it has been working

Waze staff wrote:1. A median shorter than 15M.

2. Parallel roads. The angle is determined regardless of the median. It's the angle of the start of each segment in each road relative to the north and the driving direction. In the case of parallel roads with opposite driving directions, one will have an angle of 0 degrees and one of a 180 degrees (give or take 5 degrees). This is regardless of what happens to the segment after the start point in that intersection.

To explain this a bit clearer I like to look at it as 3 rules.

1) The median segment is less than 15m.

2) The first and last segment of the uturn are one-way segments, with opposite driving directions.

3) The portion of those two segments closest to the median segment must be exactly parallel ±5°.

How is parallel measured? Look at the junction between the first segment and the median as the center of a compass, and the draw a straight line from the last portion of the approaching road through the junction to the edge of the compass. The angle it points to is the angle of that segment (we'll call it segment A).

Now position the junction between the median and the second segment at the center of the compass. Draw a straight line from the junction over the first portion of that last segment and continue to edge of the compass. That angle it's pointing at is the angle of the last segment (segment C). Now if those two angles are 180° apart (±5°), the segments are considered parallel.

The math, start with the higher of the two angles mentioned, subtract 180, then subtract the other (lower) angle. If the number you're left with is between -5 to 5, the segments are considered parallel. Sent using Tapatalk for Android 4.4.2