[Routing] Bad routing: U turns for highway links?

[Stephen Woodbridge]

Anthony wrote:
> On Sat, Jul 17, 2010 at 10:52 PM, Alan Millar <am12 at bolis.com 
> <mailto:am12 at bolis.com>> wrote:
> 
>     Am I really asking something unreasonable of a router that when an
>     xxxx_link way meets an xxxx way at a very low angle, the router should
>     know to go forward and not almost reverse?

To some extent yes, this is unreasonable to ask of the router. While it 
is not totally impossible to build a router to do this, routing 
algorithms do not deal with geometry they only deal with connectivity. 
You have to have geometry to deal with angles. While this could be added 
to the preprocessing of the data into the graph file you would have to 
represent it as another cost constraint or as a turn restriction and the 
router would need to be "aware" of how to handle these.

It is better to fix the data, period!

You can not expect all routers to implement every weird hack to work 
around all the weird data problems that various data sources have and 
might want the router to handle. This is just not reasonable.

-Steve W


> I don't know.  It sounds like a reasonable rule, but 1) is it 
> necessarily *illegal* to make these turns, or is it just generally 
> discouraged? and 2) Is it always the case?  Marcus says that "such 
> intersections with very sharp angles exist in many cities and are 
> valid".  He didn't make it clear whether these are instances of 
> xxxx_link meeting xxxx (as opposed to xxxx meeting xxxx or yyyy), but if 
> so, I'd like to see examples, and I suppose that kills this solution.
> 
> I think if you want this enacted the first step would be to go through 
> the database and provide a list of all the situations where it happens.  
> Then we can go through, take a random sample, and get an idea of how 
> often this type of turn is legal or illegal.  We also can get an answer 
> as to how widespread it is.  If the numbers are low, we should just fix 
> them by hand.  If it's high, then maybe a different solution is going to 
> have to be made.
> 
> I just found one near where I live 
> (http://www.openstreetmap.org/?lat=28.077361&lon=-82.570335&zoom=18&layers=B000FTF 
> <http://www.openstreetmap.org/?lat=28.077361&lon=-82.570335&zoom=18&layers=B000FTF> 
> any route from Alloway Street to/via Ehrlich Road).  And if you look at 
> the aerial, it's definitely not a dual carriageway.  I'm not 100% sure 
> whether or not it's a legal turn, though.  I suspect it might be legal, 
> *if* it can be done safely and without blocking traffic (i.e. when 
> traffic is low).  In which case, what is a router even supposed to do?
> 
> 
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