Funny highway shortcuts with the new routing feature

Posted by Amazed on 20 February 2015 in English (English). Last updated on 23 February 2015.

The new routing feature on will make it a lot easier to find certain types of errors in the map. I tried it out and found errors that I would have overlooked otherwise.

Autoroute 40 is a limited-access divided highway in Quebec. Crossing gaps in the median strip are used for circulation during roadworks. These gaps are currently tagged as service roads on OSM.

What route should you take if you miss your highway exit and you want to go back in the opposite direction? If you are a boring law-abiding citizen, you will have to keep driving until you can take the next exit and then re-enter the highway in the other direction. OSM’s routing algorithm, however, relying on less-than-perfect road access data, worries not about following such pointless traditions. Its free mind comes up with a more daring, if illegal, route that involves a hard 180-degree left turn through a crossing gap:

crossing autoroute 40 through gap

Clever, but a little too dangerous for my taste. Adding access=no to those service roads turns OSM back into a less audacious, but safer route planner.

Location: Deschambault-Grondines, Portneuf, Capitale-Nationale, Quebec, G0A 1S0, Canada

Comment from Rovastar on 20 February 2015 at 23:12

The should not change the map data to fix a router. That would be mapping for the router.

If it is legal to go down that service road then you should leave it as it is.

Comment from Richard on 20 February 2015 at 23:12

…is exactly why we’ve added routing to the front page!

Large amounts of OSM data is unsuitable for routing. Putting routing front-and-centre means this will get fixed. Thanks for fixing this one and I hope you find many more such bugs to fix. :)

Comment from RobJN on 21 February 2015 at 02:05

Rovastar, looking on Gmaps shows that there a posts stopping easy access, and what access is maintained is signposted as no u-turn except authorised vehicles. We could debate how best to map this, but Amazed’s solution is a huge improvement over the current situation.

With every fix like this, routing in OSM will get better and better :-)

Comment from Warin61 on 21 February 2015 at 05:37

Rovastar re read the text …

” Crossing gaps in the median strip are used for circulation during roadworks”

Implies not for other use. Thus access=no.

Comment from Rovastar on 21 February 2015 at 10:00

I still stand by what I said sigh

Comment from robbieonsea on 21 February 2015 at 10:42

Rovastar: I suppose one could map the physical barrier bollards/posts - but I agree the access rights are not for general public use and thus need to be defined better.

I probably would have used access=private, maybe adding emergency=yes would be even better. One assumes Emergency services could do such daring 180 degree manoeuvres.

Comment from Richard on 21 February 2015 at 13:30

[For the avoidance of doubt, my comment was a reply to the original posting, not to Rovastar’s comment which was posted at the same time.]

Comment from Tiliae on 22 February 2015 at 17:33

Look at the location on gMaps. There’s is clearly a turn restriction on the small service road. This is not an issue of routing, this is bad map data.,-72.129876,3a,75y,234.59h,92.37t/data=!3m4!1e1!3m2!1s92pDyj4L6nle8BUrjvSofA!2e0

Comment from skquinn on 23 February 2015 at 01:11

In the US, these types of crossings on Interstate highways are usually signed “For Official or Emergency Use Only” and for good reason.

access=no with a possible emergency=yes is proper tagging.

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