During turn restrictions mapping in various cities, we have come across certain special cases where we need to pay more attention while mapping. These cases are noted down here for your reference.
1. Restrictions on Red
Be careful with interpreting signage and always verify Mapillary signage overlay with the image and the kind of restriction. The image below shows a sign for no free right that Mapillary interpreted as no-right-turn. This is NOT a turn restriction. If you are not sure, consult the community before mapping.
A similar restriction found in Canada depicts that left turn cannot be taken when the signal is red, otherwise it is permitted to turn left. We are not adding such turn restrictions.
Picture by Share Bear - Own work - Public Domain
2. Location and bearing of Mapillary Signage overlay
The location and bearing of the Mapillary photo is not reliable and is prone to usual GPS errors. Cross check the photograph with the map to confirm you are looking at the intersection from the correct bearing. This photograph is facing the reverse of actual heading.
3. Single restriction per image detection
Mapillary will detect only one sign type per image. Lookout for multiple restrictions in the image
4. Careful with Turn restrictions plugin in JOSM
Sometimes the Turn restictions plugin does not detect the correct signs i.e. even though it's a no-right-turn, the plugin detects it as no-left-turn. So be extra sure before adding the restriction
5. Advantage of previous/next images
There might be an offset in the Mapillary images, that is the image may appear after/before the actual location. It is ideal to inspect the previous and next images and also look for road names and other clues from the satellite imagery to verify it's exact location and bearing.
6. Temporary turn restrictions
When the roads are in construction or under repair, you might see temporary turn restrictions placed similar to shown in the below image. We are not adding temporary turn restrictions.
7. Restriction-ahead warnings
In the below image, there is a no_left_turn sign board as detected from the Mapillary image. This sign board means that there is a no_turn_restriction ahead. But this signage itself is not a turn restriction, so we do not map this.
Case:1
The image below shows two signages, the first one is in French and translates to 'Wait for arrow to turn left' and the second one is a no_u_turn. In such cases, we ignore the the first signage and go ahead mapping no_u_turn.
Case:2
In the below image, the first board for no_left_turn says Monday to Friday 8am-10am and 3pm-7pm (Lun-Ven 8h-10h,15h-19h in French). The second board is a no_u_turn except authorized vehicles. In this case, first one would be a conditional turn restriction; second, an absolute no_u_turn turn restriction.
Case:3
The below image indicates that all the drivers (no matter the lane they are on) must follow this/these direction(s) at the intersection. This means that the driver needs to only turn right/left on the days mentioned in the sign and during that duration of the day.
Syntax to be used: restriction:conditional=only_right_turn @ (Mo-Fr 07:00-09:00,16:00-18:00)
In Germany, instead of signages to prohibit turns, the signages describe the legally allowed turns at the junction. Hence a signage for a no right turn will indicate only turns for left and straight.
An example of a restriction=no_right_turn traffic sign in Germany
Courtesy: German wiki