[Tagging] Micro mapping traffic signals

Kytömaa Lauri lauri.kytomaa at aalto.fi
Wed Aug 28 11:39:15 UTC 2013

Pieren wrote:
>was placed on the intersection node itself. 

>routine engine where routes with traffic signals 
>are penalized. 

I won't be saying anything about the discussed 
alternatives at this time, but just wish to point 
out that "this intersection is controlled by signals" 
when used only on the intersection nodes, can't 
be straight away used as a constant time penalty 
for each such node.

For example, where two dual carriage way roads 
intersect, (right hand drive) 
- a route turning right passes one node tagged 
with highway=traffic_signals
- a route going straight goes through two such
interesction nodes
- a route turning left goes through three such

For some even more complex intersections where
carriageways are further divided before the signals,
the "wrong count" can be and would be even higher.

Even where a single carriage way road crosses a dual
carriage, the other road's traffic going straight passes
through two such nodes - the other road's traffic only
through one such node. I'll let everyone to consider
by themselves how the "passed node count" 
changes on different routes through the intersection,
if mappers happen to move the tags away from the
intersection node.

This shortcoming exists for all intersections 
where not all roads are single carriageway roads,
in different ways. It always need more data, or 
algorithmically derived guesses. Only when the 
intersecting roads are both single carriage twoway
roads, tagging the signals just before the intersection
doubles the "constant penalty" effect, if a router uses
the nodes blindly. IMO, therefore, this "time penalties 
go all wrong" can't be used as a reason why it's always
more correct to tag the intersection node.

I'd be happy to see someone explain here how their
router does something more complex with the 
highway=traffic_signals nodes.


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