Project of the week/2010/Aug 29
Turn Restrictions
One of the most frustrating things about navigating a vehicle through urban areas are turn restrictions. You want to go right but the sign says you must go left. Some of the frustration is caused by the additional driving time and increased number of turns required, but that frustration is compounded if you didn't know that the turn restriction existed in the first place.
With well mapped turn restrictions, you can plan a better route right from the start, rather than adapting later. The project of the week is to check and update turn restrictions in your area.
Turn restrictions have already increasingly become popular, with the number of restrictions in e.g. Germany growing by 63% in the last 4 month. So lets make sure they become a first class citizen in the OSM tagging world!
Monitoring
You can explore the OSM world of turn restrictions using these maps:
One-way roads note
You may see "no right turn" signs () which do not need to be mapped as a turn restriction (relation) in the OpenStreetMap data, because they are implied by one-way roads. It's easy to tag a one-way road. Just use the oneway=yes tag and ensure that the way is oriented in the correct direction. With this in place, it is often not necessary to add a turn restriction relation (described in the following section), since the turn restriction is implied by the one-way. Clearly you can't turn right into this road because you're not allowed to go in that direction down the road at all.
Turn restrictions must be mapped more explicitly in cases where you are allowed to drive down the road, but not allowed to make a turn from one particular direction.
Method
Adding turn restrictions is not all that easy because it involves adding a relation, but you will be guided by most editors.
Potlatch
Potlatch 2
Potlatch 2 has a dedicated turn restrictions editor.
- Open Potlatch 2 at www.geowiki.com
- Select the node at the junction (it will need to have at least two ways ending there)
- Select the 'Restrictions' tab on the left
- Add your restriction!
JOSM
Video-Tutorial assisted with JOSM/Plugins/Turnrestrictions
Mapzen
It’s a piece of cake to edit traffic flows in Mapzen, thanks to the new Junction Editor.
- Highlight the node on the road, intersection or cross street and
- Select the ‘Edit Turn Restrictions’ button located at the bottom right
- A window appears with cars traveling in each of the directions allowed at the intersection
- Add/edit you restriction by clicking on a car and on the corresponding arrow or no-entry symbol until it reflects the correct traffic flow
Junction Editor from Mapzen is in Beta version now, if you want to try it - send a request to mapzen@cloudmade.com
Mapping
All in all this is just an relation grouping all affected roads together:
- type=restriction and adds a category e.g.: restriction=no_right_turn
- Then the member roads have become the roles from,to and the crossroad node as via role
See also: Turn restrictions
Routers supporting Turn restrictions
Turn restrictions have always had a bit of a chicken and egg problem. People didn't map turn restrictions, as many routers didn't support them. Vice versa, they have often not been implemented in routers, as there were too few in the data. So let's make sure we add enough restrictions to make it an essential feature for any OSM router. And indeed, increasing numbers of routers do support turn restrictions by now, making it well worth mapping them already! Below is a list of those routing engines that support restrictions and that you can use to check the restrictions you have added (note that these systems may not pick up your change for several days)
Online routers
- YOURS navigation - Webinterface to the Gosmore routing engine.
- Cloudmade routing
See also Routing Compare
Mobile applications
- Routable Garmin maps
- GpsMid - Java ME base offline map application supporting navigation including turn restrictions.
- Skobbler (Using Cloudmade routing) - Online turn-by-turn navigation for iPhone and Android
There are bound to be more routers supporting turn restrictions. Please add them here if you know any.