[Talk-us] Prima Facie Speed Limits

Tod Fitch tod at fitchdesign.com
Mon Sep 15 19:37:25 UTC 2014

On Sep 15, 2014, at 1:09 AM, Paul Johnson wrote:

> On Sun, Sep 14, 2014 at 6:52 AM, Tod Fitch <tod at fitchdesign.com> wrote:
> maxspeed:source=* was a "thinko" (kind of like a typo but different part of the body). :)
> OK, probably, but it does sort nicely in JOSM.  Either way, feel free to fix my mistagging on this...I've used maxpseed:source=* on a few counties now and if I got the key backwards, feel free to fix.  I'm not going to argue over extremely minor semantics like this.

Interesting. I happened upon source:maxspeed=* and found it in the wiki so went with it and there are over 455,000 uses of that tag. But there are over 22,000 occurrences of maxspeed:source=* which is quite a high number too. Of those 9,301 are maxspeed:source=default␣residential␣speed␣limit␣in␣Australia so it looks like the use is global and that taggers outside of the US are also dealing with default or prima facie speed limits.

My take on the opinions in this thread are:

1. maxspeed=* should have a speed value that is of use to a router or navigation aid. So a number (for KPH) or number with " MPH" suffix are greatly preferred over things like "maxspeed=unposted".

2. The only real way to get a maxspeed=* value is by survey. Even if the jurisdiction has a default speed limit for the type of road, any specific road may be signed to some other speed. And, in general, you can't see signs in satellite imagery. (Exception in California and perhaps elsewhere: Occasionally the speed limit is painted on the pavement and that can be seen in satellite imagery.)

3. A source:maxspeed=* tag should accompany any maxspeed=* tag indicating the authority. Usually that will be source:maxspeed=sign. Since this tag is for human documentation purposes and probably will not have a software based data consumer values can be something agreed to by the taggers in the area. For unsigned roads things like source:maxspeed=DE:urban seems to work for German taggers, but something like source:maxspeed="California vehicle code 22352" makes it easy for anyone to do a web search and determine the criteria. source:maxspeed=survey is fairly widely used but that does not say if the maxspeed was from a sign or a jurisdictional default so I would avoid using it.

4. There are unsigned roads where the jurisdictional boundaries are unclear and the jurisdictions in question have different default speed limits. So the actual speed limit is not clear to the tagger doing the survey. (Neither is it clear for drivers on that road which can lead to issues in enforcement but that is not on topic.) In that case the maxspeed=* value is unknown and the tag should not be used as it adds no value for the intended data consumers. But we want some way to show that the road has been surveyed and the speed is really unknown on the ground not just unknown in the OSM database. I think a fixme=* tag with explanation in the issue would handle that.

I've been off on other things so I haven't done more than make sure that the roads tagged with maxspeed=* that I have done in my area also have a source:maxspeed=sign. I think I am comfortable enough with this discussion thread that I will add maxspeed="25 mph" and source:maxspeed="California vehicle code 22352" to the unsigned residential roads I have surveyed.

Thanks for the input and discussion!

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