Improving routing information using turn restrictions

Turn restrictions, like No left turn and No U turn, regulate traffic flow at intersections and accurately mapping them is critical to calculating valid routes on a map. The Mapbox Directions API uses OpenStreetMap which means you can easily update turn restrictions yourself — here’s how.

No right turn sign. Photo: pdd.ua

Turn restrictions in San Francisco (fullscreen map).

You can edit turn restrictions on OpenStreetMap with both the built in iD editor or the desktop based Java OpenStreetMap editor (JOSM). We’ve already covered mapping turn restrictions with iD so I’ll focus on JOSM here.

Mapping turn restrictions with JOSM

Consider a scenario where there is No Left turn allowed from North Beverly Drive to Dayton Way

1. Install plugin

To get started in JOSM, download the turnrestriction plugin in Preferences > Plugins.

Installing the turn restrictions plugin.

2. Select ways

Next, select the ways involved in the turn restriction: North Beverly Drive and Dayton Way.

• Split the road at the node where the turn-restriction will be added.
• The selection order of the ways is important in order to determine the type of restriction. Select North Beverly Drive (from) then node at intersection (via) then Dayton Way (to).

3. Select type of restriction

Finally, select the type of turn restriction to add. The plugin will detect the most appropriate restriction based on the order of ways selected. Done!

Behind the scenes, this applies the appropriate relations and tags that compose the turn restriction. Read more on how turn restrictions are modeled and how to add complex attributes like temporal restrictions on the OpenStreetMap wiki.

Adding a turn restriction using the turn-restriction plugin: select ways, then select type of restriction.

Here’s an overview of some of the tag combinations describing turn restrictions in OpenStreetMap:

Symbol	Description	OpenStreetMap Tag
	No straight through	restriction=no_straight_on
	No Right Turn	restriction=no_right_turn
	No Left Turn	restriction=no_right_turn
	No Turns	restriction=only_straight_on
	No U-turn	restriction=no_u_turn
	No Left or U-turn	restriction=no_right_turn;restriction=no_u_turn

Join mapping!

We’re adding hundreds of turn restrictions in San Francisco, Boston and Washington DC using Mapillary, Improve-OpenStreetMap and telemetry data. You can join our work directly on Github and if you have questions or more ideas for mapping turn restrictions, find me on twitter.