[Talk-us] Does only_straight_on imply no_u_turn?

[Richard Welty]

On 1/8/10 12:59 PM, Randy wrote:
> 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.
actually, i think there is a bit of discussion on rendering on the turn 
restriction relation page,
and i think it is to a degree wrongheaded as it shows equvalencies 
between the red "can't" and
blue "can" signs that aren't correct if there are oneway tags in place.

JOSM renders with the blue "can" signs, and ignores any underlying 
oneway tags on the
ways, with the result that we can clearly see the sometimes wrongheaded 
rendering that the
Turn_restriction wiki page calls for.

since it's still apparently a proposal, i suggest we should amend the 
discussion on it and see
about getting this fixed.

richard