User:Larry Gilbert/Controlled intersections
WORK IN PROGRESS
Controlled intersections are those that are posted with a stop sign, yield or give-way sign, or a signal light (fixed stop light, programmed stop/caution/go light, etc.).
OSM already has a well-established highway=traffic_signals tag for signal lights. Here we propose and explore the idea of new tags highway=control and control=* in conjunction with a new relation, 
type=control.
Explanation of scheme
- The controlled intersection is a common node shared by two or more ways.
- The node must have the highway=control tag for identity.
- The node should have a control=* tag unless there is more than one type of control at the intersection.
- A control relation is made for each type of control at the intersection:
- type=control
- control=stop_sign, control=fixed_stop_light, control=give_way_sign, etc.
- Members include the node where the control is placed, and one or more ways that end at the node and are being controlled
- There must be no more than one relation for each type of control at an intersection.
- A relation may be omitted if there is only one type of control at an intersection and it applies to all ways that meet at the intersection.
Backwards compatibility
The following will be considered equivalent for the purposes of transitioning old data to this scheme:
| Traditional | Treat as |
|---|---|
| highway=give_way | highway=control control=give_way |
| highway=stop | highway=control control=stop_sign |
| highway=traffic_signals | highway=control control=traffic_signals |
Any of the above that are not part of a relation will be treated as controlling all ways connected to the intersection.
A node with a traditional highway=* tag may have a new control=* tag and a new type=control relation associated with it. (Once there is community consensus that enough software is compatible with the new scheme, then the old tags should be converted to highway=control.)
Examples
These examples assume a four-way intersection of two roads A and B. The ways representing the road segments are A1 and A2 for road A, and B1 and B2 for road B. X is the node marking the crossing.
Two-way stop
Travelers on road A must stop. B has no stop.
- X: highway=control + control=stop_sign
- Relation: type=control + control=stop_sign; relation members: X, A1, A2
One-way stop (oncoming traffic does not stop)
Travelers on road A must stop in the northbound direction (A2 to A1); they do not have to stop in the southbound direction (A1 to A2).
(Only A2 is included in the relation because people traveling on A2 toward X must stop at X.)
Fixed red/amber light combo at crossroads
The intersection has a four-way flashing light that is red for A and amber for B. In the United States, a fixed flashing red light means "stop fully, then proceed with caution" (same as a stop sign), and a fixed flashing amber light means "proceed with caution (without stopping)".
- X: highway=control
- Relation 1: type=control + control=fixed_stop_light; members: X, A1, A2
- Relation 2: type=control + control=fixed_caution_light; members: X, B1, B2
Edge cases
Both a fixed stop light and a stop sign in the same direction
They mean the same thing. Just use control=fixed_stop_light. That is the first thing the driver is going to see anyway. :-)
Stop sign marked "Right turn does not stop"
This is not accurately captured by the scheme in its current form.
Perhaps a kluge could be worked out using a turn restriction as an "anti"-restriction, but this is overloading the meaning of a turn restriction and is probably undesirable.