Actually, this is a sharp right turn, not a u-turn!
You run a route through Match to Roads, open the turn list, and step three says U-turn onto Bareldonkstraatje.
It is not a u-turn. It is a sharp left at a fork, the sort of corner you take without really slowing down, and we have just told the rider to stop and turn round.
That report came from a reader who has been through more of our GPX output than anyone else, and who knows the Locus Map and Cruiser navigation extensions from the user's side far better than we do. They were right, and the rule turned out to be more interesting than I expected.
Three symbols, not one
The navigation extensions those apps read do not have a single u-turn. They have three.
I had been treating the first as the vague version of the other two: it says turn round, and if we know which way the road bends we may as well say so. That is backwards.
Cruiser shows a u-turn as an inverted U without arrows. Locus shows a u-turn as an inverted U with 2 arrows. This means the user practically FREELY determines whether to turn left or right and come back. With a left or right u-turn you MUST turn as the icon indicates.
The plain symbol is a permission. It appears where you go back down the road you arrived on, and it does not care which way you swing round, because either way works. The directed symbols are instructions: there is a correct way round, and the other way puts you somewhere else. Turning every plain u-turn into a directed one, which is what I did first, does not add information. It takes away a choice the rider had.
The angle cannot tell them apart
We read corners off the shape of the line: the heading 20 m before a point against the heading 20 m after it. Past 150 degrees is u-turn territory.
That gets you into the neighbourhood and no further.
| where | angle | what it should be |
|---|---|---|
| Kurviger route, point 216 | 151 degrees | sharp left |
| Kurviger route, point 22 | 156 degrees | u-turn left |
Five degrees apart, opposite answers. No threshold fixes that, because the angle is not measuring the thing that makes a u-turn a u-turn.
What actually decides it
A u-turn is about where you end up, not how far round you went.
So we measure the gap between the way in and the way out. Under 5 m the two are the same road, which is the plain symbol and the permission that goes with it. Under 25 m, near enough a carriageway plus a verge, so it is worth one instruction instead of two consecutive lefts, but you have to go round the correct way and the symbol has to say which. Beyond that it is a different road, and a sharp turn.
That is the middle case on a real map. The route goes out along Sportlaan and comes back on the road immediately alongside, so both turns are u-turn_left: not a choice, an instruction.
Except the gap does not work alone either:
| where | angle | separation | what it should be |
|---|---|---|---|
| Kurviger route, point 216 | 151 degrees | 35.4 m | sharp left |
| Cruiser's own file, point 3 | 175 degrees | 34.3 m | u-turn left |
One metre apart. The first is a fork onto another road; the second is a genuine turn round that uses a wide bit of tarmac to do it, and Cruiser itself labels it u-turn_left, so there is no arguing with it.
Plot every near-reversal in the three test files against both measurements at once and you can see the shape of the answer:
No vertical cut separates the colours and no horizontal one does either. What works is the corner: past 150 degrees and coming back close, or past 170 degrees regardless of how wide the turn had to be. Beyond 170 you are so nearly reversed that a wide sweep is just a wide sweep; below it, the width is telling you that you have gone somewhere else. Four numbers, all sitting together at the top of the file so they can be argued with:
uTurnDeg: 150, // in u-turn territory at all
sameLineM: 5, // the same road: plain u-turn
parallelM: 25, // a carriageway and a verge: directed u-turn
reversalDeg: 170, // so nearly reversed that width stops mattering
Measuring the gap
Two traps in there. The first is the median: it is tempting to take the smallest distance between the two legs, but they meet at the turn itself, so the smallest is always about zero. The second is that the exit points are measured against the approach line, not the nearest recorded point on it. Point to point, the answer depends on how densely the file happened to be logged, and a track recorded every second would score differently from the same road recorded every 50 m. That is not a difference anybody riding it would recognise.
The smallest test file caught a related bug. A single 180 degree turn, one road, 13 points across 673 m, which is about 90 m between vertices. The probe walked 60 m either side looking for exit points, found one, and stopped. One point defines no line, so the turn fell through unjudged. Collect two points first, then check the distance.
Why it is red
U-turns are flagged red in our turn list, because an unexpected one nearly always means a shaping point landed on the wrong side of a road and the router has dutifully sent you past it and back.
Here the route doubles back on itself, so the symbol is the plain one and its icon carries no arrow. Turn round whichever way you like.
That flag was keyed on the literal string u-turn, so the moment we started emitting u-turn_left it would have quietly stopped appearing on the turns most worth flagging. It now runs through one shared test that knows all four spellings.
The icon
Bootstrap Icons, which we use everywhere else, has no u-turn. It has no acute sharp turn either, and nothing that says bear left at a fork without also saying or bear right. Turn instructions have moved to a proper manoeuvre set, and we drew the side-neutral u-turn ourselves: an arch with an arrowhead at each end, which is as close as a static glyph gets to saying "either way". The whole vocabulary is on turn-icons.html, in both themes.
Where it landed
All three test files now convert to exactly what their sources say. The Kurviger route yields no plain u-turns at all, because it contains no true reversals. Cruiser's own file keeps both of its directed ones. The 180 degree demo stays plain, with the choice it was always offering.
That last part is the bit I would not have reached on my own. A plain u-turn is not a directed u-turn with the direction missing. It is a different instruction, and the rider is the one who decides.
Header photo by Jim Wilson on Unsplash.
