Turn restrictions – a vital part of any routing system

The best part of using everyday OSM technologies and relying on OSM to make sure that you get “there” on time is that you can directly influence the quality of the experience.

Regardless which OSM technology you’ll be using, to provide you the best experience possible, the routing software has to know as much information as possible about the roads between you and your destination: one-way streets, turn restrictions, speed limits, road closures and much more.

For example, the turn restrictions contribute significantly to the total travel time, and to the correctness of the route altogether, thus, by ignoring them in the traffic network model, essential characteristics of the network might be missed, leading to substandard and unreasonable paths.

Dealing with turn restrictions in OSM

To help us navigate the complexities of properly translating real map scenarios to the ways and points schema of OSM we will rely on JOSM with the turn restrictions plugin installed.

Turn restrictions in OSM are handled by creating a relation

A relation is one of the core data elements that consists of one or more tags and also and ordered list of one or more nodes, ways and/or relations as members which is used to define logical or geographic relationships between other elements. (source)

There is a mandatory requirement when creating a turn restriction relation: it has to consist of minimum three members and must have assigned two tags. (see below example)

structure
The ‘type=restriction’ flags the relation as a turn restriction and ‘restriction=no_u_turn’ indicates the restriction type.

A ‘no_’ type relation can also be represented in map data as an ‘only_’ type relation. The prohibited turn restriction relation is preferred by some routing engines instead of an allowed turn restriction relation.

More details here - https://wiki.openstreetmap.org/wiki/Relation:restriction
More details here – https://wiki.openstreetmap.org/wiki/Relation:restriction; US regulatory signs – http://mutcd.fhwa.dot.gov/services/publications/fhwaop02084/

Members of a turn restriction relation are ways and nodes

One simple case can be a turn restriction relation that consists of three members – two ways and one node. The two ways would represent the beginning (‘from’ role) and end (‘to’ role) of the turn restriction. The node would represent the continuity of travel between two ways and has a ‘via’role.

Way (A) - node (B) - way (C) sequence
Way (A) – node (B) – way (C) sequence in a ‘no_left_turn’ restriction relation.

Another case is where a turn restriction relation can consist of three or more ways. Two ways from this type of relation would represent the beginning and end of the turn restriction and at least one way would represent the continuity of travel between the aforementioned ways (‘via’ role).

Way (A) - way (B) - way (C) sequence in a no_u_turn restriction relation
Way (A) – way (B) – way (C) sequence in a ‘no_u_turn’ restriction relation.

Workflow for adding turn restrictions

The traditional way

Using the embedded relation editor available in JOSM. A slight disadvantage of this method is that you spend a bit more time to manually construct the relation. Click on the image below for how-to video.

traditional_way_vid

The user-friendly way

Using the turn restrictions plugin, that automatically recognizes the type of relation and roles for each member. Click on the image below for how-to video.

user_friendly_vid

Using the aforementioned tools, we have reviewed 2,000 miles of field trip footage and added nearly 2,500 turn restrictions in the LA/Orange county area, where 85% of the turn restrictions that were added to the map are no_u_turns, followed by 11% of no_left_turns, the rest being covered by the other categories.

Hopefully we’ve managed to illustrate how easy is to map turn restrictions in OSM. Now, it’s your turn!

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Author: mihai

Get in touch with me at twitter.com/ubermih

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