this post was submitted on 31 Aug 2024

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Offset speed limit signs: am I mapping this correctly? (lemmy.ca)

submitted 2 months ago* (last edited 2 months ago) by [email protected] to c/[email protected]

13 comments fedilink hide all child comments

I have recently got into mapping traffic signs and speed limits where I can, but I realised I don't know what to do when mapping differing speed limit signs that are offset from each other. It's a transition from a 50km/h zone to a 60km/h zone but the signs are offset by 65m, making a sort of "no-mans land" in-between. I plan to map it with the in-between zone tagged with separate maxspeed:forward and maxspeed:backward tags. I assume it should be done this way because as a driver on the ground I would follow the speed limit signs on my side of the road as I passed them, having no knowledge of the reverse sign.

I guess I'm wondering if this is a logical way to map this?

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[–] [email protected] 8 points 2 months ago

Couldn't you just create dedicated tags for the individual directions of the lanes? https://wiki.openstreetmap.org/wiki/Key:maxspeed mentions this in the Lanes section.

[–] [email protected] 8 points 2 months ago* (last edited 2 months ago) (2 children)

I plan to map it with the in-between zone tagged with separate maxspeed:forward and maxspeed:backward tags.

Maybe it would be better to use the lower speed limit in the "no-man's land", since it would be unfortunate to be driving too fast while thinking one is folliwing speed limit. I assume it's kind of ambigous what the allowed speed limit is?

For mapping, I agree with your logic. I just think we should err on the side of causion when things are ambigous.

[–] [email protected] 13 points 2 months ago (1 children)

I think what he means is, from the point of the driver driving from the 50 to the 60 area:

You drive fifty, then you pass a sign pointed away from you (towards oncomming traffic) saying 50. 65m further, you pass a sign pointed at you saying 60.

In that case I would say the speedlimit is just different for that 65m stretch of road, based on the direction of travel. The speedlimit changes at the 50 sign if driving from 60 to 50, and at the 60 sign if driving from 50 to 60.

[–] [email protected] 5 points 2 months ago

If I misunderstood, and this what OP meant, then I agree with your reasoning and solution.

[–] [email protected] 5 points 2 months ago

Fair point! Better safe than sorry. On the other hand, drivers 'round here will go 20 over no matter what I designate it as lol. Kidding... kinda.

[–] [email protected] 3 points 2 months ago

The speed limit need not be the same for each direction of travel. The speed limit changes at the sign. There is no expectation that the signs be at the same road stationing for opposing traffic. If the open street map database can't handle that appropriately, then there is no correct way to map this situation.

[–] [email protected] 2 points 1 month ago (1 children)

@sven @openstreetmap I think the tagging you're showing *should* be fine, but as I think the :forward and backward tags aren't as widely supported I'd put the lower speed as a generic maxspeed for that bit of road as well. That way simple software will default to the lower limit and more sophisticated software will use the correct one.

[–] [email protected] 1 points 1 month ago

That's a good point! Keeping compatibility is important.

[–] [email protected] 1 points 1 month ago

Interesting. I just found this instruction. I'm guessing you already did too. (Source)

For speed limits depending on the way direction, see maxspeed:forward=*.

So I would say, map that section of road with forward/backward speeds. Also, the signs should be independent nodes.

The position of the actual speed limit signs can be added as nodes beside the road/railway/waterway

[–] [email protected] 1 points 2 months ago (2 children)

As far as I know. When speed limit goes UP, the speed limit takes place as soon as the sign is visible. But, When the speed limit goes DOWN the limit is in place at the location of the sign and beyond

[–] [email protected] 3 points 2 months ago* (last edited 2 months ago) (1 children)

Heh, wouldn't that be nice. No. Up or down, the default is at the sign. Only exception is inside a municipality with a rule saying signs within so far of a corner take effect at the corner.

Ninja edit: a word or two

[–] [email protected] 1 points 2 months ago

I imagine this is different per country as well? I've never heard of municipalities with that rules in my country, at least.

[–] [email protected] 1 points 2 months ago

I always assumed this to be a joke in reference to driving too fast.