[Talk-us] Does only_straight_on imply no_u_turn?

[Randy]

Alan Mintz wrote:

>When tagging a T-shaped intersection like this:
>
>     B
>     B
>     B
>     B
>AAA+AAA
>
>if you tag a no_left_turn restriction from A into B, it renders (in JOSM
>anyway) as an arrow going straight and right. This doesn't make sense in
>these T-intersection cases because there is no road continuing right from
>the intersection. So, I've been tagging these as only_straight_on instead,
>which effectively is the same thing, but renders better (though not the
>same as the sign).
>
>However, in tagging a no-left/no-U at such an intersection, I realize that
>"only_straight_on" might also imply no_u_turn, in which case most of my
>previous tagging in the case above is wrong, since most of them were
>no-left-turn signs only (i.e. U-turns are allowed).
>
>Does only_straight_on imply no_u_turn (in addition to meaning no left and
>no right)? If so, does it make more sense to render no_left_turn as a
>left-turning arrow with a line through it, instead of the current
>straight+right arrow?

I would say this is a very good example of don't tag for the renderer. The 
information you have given is adequate for a renderer to properly label 
the intersection (no left turn + no highway to the right). A renderer 
should be able to take this (not so) special case into consideration in 
labeling the flow directions. While only_straight_on is also valid, since 
it is the only available option when no left turn is allowed, consider 
this: If the the N-S highway, in your example, were ever extended, then 
someone would have to notice the restriction in the  relations and change 
it or right turns wouldn't be routed. Better, in my opinion, to stick with 
the signage.

I would not count on only_straight_on as being interpreted as no_u_turn; 
nor would I assume no_left_turn includes no_u_turn as a default. 90% or 
more of the time the implication would be there, but there are some cases 
where a U-turn may be permitted at a traffic light, against the designated 
traffic flow. Much better, and more flexible, to overtly include all 
restrictions.

-- 
Randy