It isn't always obvious where to position a highway=motorway_junction node that connects a motorway way to a motorway_link way (also known as an off-ramp or slip road). Over the years, mappers have used three different approaches.
Inconsistent placement of junction nodes can affect turn-by-turn navigation software, particularly instruction timing and rerouting. I'd like to raise awareness about the preferred placement, which is at the beginning of the gore, and explain why the other two approaches are suboptimal.
Throughout this post, I will refer to the term gore. A gore is a wedge that separates a ramp from the main motorway. A physical gore may be a barrier or a grassy area, whereas a theoretical gore simulates this separation using pavement markings and empty space. By “gore”, I am referring to the beginning of the theoretical gore, if present, or alternatively the physical gore.
PS: after writing this diary we had a great discussion. See comments below for the recommended “Kreuz Köln-Süd” mapping style.
Approach 1: Map the ramp node at the exit sign
Rationale: In the majority of urban areas a detailed overhead exit sign is located at the last point where the off ramp maneuver can take place - the physical or theoretical gore.
- Many rural exits (and some urban exits, depending on the state) lack overhead signage. Instead, the only sign is located on the side of the road (at the beginning of the deceleration lane or even earlier), accompanied by a small “Exit” sign at the gore.
- In many cases, the exit sign occurs well after a lane change restriction (change:lanes) begins (example - note the solid white line). However, lane issues should be remedied through tagging, not geometry alterations.
Example. In this case the exit sign and the theoretical gore are located at the same location. This is the last location where driver can exit the motorway. When the sign board is right next to the gore this approach makes sense.
Example. The exit sign does not coincide with the theoretical gore in this case. The exit sign is positioned much earlier than the gore. In this case mapping the exit from the sign position will not make sense.
Approach 2: Map the ramp node where the road begins to widen
Historically, some mappers have preferred to start the ramp at the point the road widens, mainly for aesthetic purposes.
Rationale: This appears smoother on a map rendering as it more accurately represents the path a driver should take to use the ramp.
- If a driver has passed the point where the road begins to widen but has yet to reach the gore (or the start of chevron markings), it’s too early to reroute the user as if they missed the exit.
- The exit may have a longer deceleration lane. If the junction node is placed at the beginning of the deceleration lane, the user’s distance to the junction node may differ from signage, creating confusion.
- Does not accurately reflect the features on the ground.
Example (same junction as Approach 1 example):
Approach 3: Map the ramp node at the gore of the main road and ramp
This is the approach documented on the wiki. It balances the needs of renderers and routers.
- Allows for precise lane mapping, as the junction node reflects reality.
- Staggered exit lanes should be modeled using turn:lanes tags instead of separate ways.
- The junction node reflects the exact point where it is no longer possible to cross back onto the highway, consistent with the practice of mapping a dual carriageway only when there’s a definite separation.
- On rendered maps, the turn angle looks sharper than expected when zoomed way in.
- Less sophisticated routers may consider the actual turn angle to be much sharper than in reality.
- In some cases, the gore begins well after a lane change restriction (change:lanes) begins (example - note the solid white line). However, it’s better to add the lane change restriction as a change:lanes tag than to draw the entire exit lane as a separate way.
In approach 2 the ramp node is placed where the road begins to widen. In the same example, if approach 3 is used the ramp would appear like this:
Example: The junction node near the gore where it not possible to change lanes back to the highway.
Although this approach tries to place the junction node close to the gore, it isn’t necessary to make it exactly flush with the gore. The motorway_link would meet the motorway at a 90° angle, which would result in unintuitive rendering. A roughly 45° angle strikes a better balance between the needs of renderers and routers.
Based on this evaluation, we believe the best practice for mapping ramps is (Approach 3) map the ramp node at the gore of the main road and ramp. This follows the wider OSM convention of prioritizing mapping based on the physical barriers, is appropriate for diverse geographies, and aligns with developing work around lane guidance.
As next actions, from now on, the team at Mapbox will fix these ramp geometries by following the convention stated in approach 3 when we come across ramps mapped according to other approaches. It would be great to have everyone follow this approach as well when mapping ramps and make changes to existing ramp intersections that you come across. If there is consensus around this approach let’s update the wiki with further details to promote this practice.