[Talk-us] Freeway exit tagging

Jesse B. Crawford jesse at jbcrawford.us
Fri Aug 26 20:17:33 UTC 2016

Where I live (San Francisco, CA, USA), lane arrows are generally added
for one of two reasons:

1) It isn't obvious what the authorized use of a lane is, when there is
e.g. a right, slight right, and through option with various lanes for
each - especially when there are multiple slight_rights as sometimes
2) To provide advanced warning that a lane you're in will have limited
options in the future (such as "HWY ONLY" written in lanes). The
location of these warnings varies, they're sometimes just yards before
the intersection and sometimes a block or two back - presumably
depending on how easy a traffic engineer thinks it will be to change lanes.

Lane arrows are only present in some cases though - and in my
cross-country driving experience, I have found California to be
unusually thorough about lane arrows, so they're probably present more
of the time here than they are in much of the rest of the country. For
example, lane arrows to indicate a merge_to_left are virtually never
used in the state of Oregon (this may even be ODOT policy?), but are
almost always present in California.

This is just a difference between two neighboring states in a country
with a reasonably precise traffic control manual. Lane arrow practices
are presumably even more different between countries (e.g. in the parts
of Mexico I visit, I don't think I've ever seen one).

My point is this: the practice of marking lanes with arrows varies by
jurisdiction and by how recently the intersection has been painted. In
many cases, in the US at least, it depends on whether the intersection
is straightforward to navigate for humans or not. It probably also
depends on whether or not violations have been observed there - here, at
least, they sometimes respond to too many accidents at an intersection
by putting in physically larger stop signs. I suspect the same is done
with land arrows.

Do any of these factors in road markings change the actual ground truth
of the intersection and its navigation?

I think this is the point that Rihards was getting at, somewhat
hyperbolically. Road markings like lane use arrows and signs are usually
themselves representations of the intersection to come, much like a map.
When we produce a map, we shouldn't be making a map of someone else's
map of the intersection. We should be reflecting the way the
intersection actually is. Which, yes, is sometimes designated only by
restricting signs, but often it is not, and is instead restricted by
standard rules about the usage of lanes (i.e. no right turns from a lane
other than the right curb lane, UNLESS markings indicate otherwise).
These rules, too, vary by jurisdiction, so if the map is going to be
general for global use it should express them.

Jesse B. Crawford

jesse at jbcrawford.us
GPG 0x4085BDC1

On 08/25/2016 11:30 PM, Paul Johnson wrote:
> On Thu, Aug 25, 2016 at 4:28 PM, Rihards <richlv at nakts.net
> <mailto:richlv at nakts.net>> wrote:
>     On 2016.08.26. 00 <tel:2016.08.26.%2000>:15, Jack Burke wrote:
>         Freeway exit tagging
>         I am totally confused.
>         What is the proper method to use turn:lanes to tag freeway lanes
>         approaching an exit, where the exit branches directly from an
>         edge lane
>         without being part of the freeway itself, but the freeway lanes
>         are not
>         signed with an arrow, such as this one?
>          http://mapillary.com/map/im/7igAGXSa6EsUYlTIujXchw
>         <http://mapillary.com/map/im/7igAGXSa6EsUYlTIujXchw>
>         Through examples[1], the wiki shows that when the freeway lanes
>         *are*
>         signed, then "through;slight_right" appears to be the correct value.
>         The wiki examples also appear to indicate that "through" is *only*
>         appropriate when there is corresponding signage.  The wiki is
>         also very
>     referencing the previous topic in talk-us about how lane tagging
>     should follow lane _markings_, i'd like to suggest to only map the
>     legally allowed driving directions, no matter how we arrive at them.
>     mapping the road markings seems extremely strange - what if they are
>     very faded, when do we map them ? is there a threshold of % of the
>     paint left ?
>     what is there are no road markings but there are signs ?
>     do we remove those tags during the winter in some regions ?
>     mapping of markings separately also seems to have no functional
>     benefit. the information should be useful for navigation software -
>     or, more importantly, for the end user (no matter which software
>     delivers useful service to them). they don't really care how exactly
>     the allowed directions are marked, as long as they get through it
>     all without crashes and fines.
> This is a pretty nice summary of what I was getting at in the previous
> thread on talk-us.  Especially given how common it is to not have the
> turn arrows in the first place.  I'm a firm believer that the wiki is
> wrong on this one and the ground truth for turn lane tagging should be
> the actual application of the lanes, with or without the presence of
> supporting signage or pavement markings.
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