Relations/Proposed/Bridges and Tunnels

This is a proposal for a set of tags and members making a Relation to represent a bridge or tunnel.

This does not replace bridge=yes/tunnel=yes attached to a way. That is still the simplest and easiest method to represent a straightforward bridge carrying a single way.

The Bridge/Tunnel relations:

• group together ways which share a common bridge: for example, a dual carriageway (divided highway) where both carriageways pass over the same bridge structure. In the absence of a bridge outline, a renderer would show bridge parapet symbols alongside only the outermost constituent ways

• allow the possibility that a bridge or tunnel may be independently named from the ways which it carries.

• allows both the way(s) crossing the structure and those passing under it to be identified (river and most canals bridges only pass over the waterway, and using only the Way tag bridge=yes means that intersection tests would have to be used to determine the bridges; now we can associate the bridges with the waterway as well.

• allow for properties of the structure to be represented (e.g. clearance).

• optionally identify a way showing the outline or "footprint" of the bridge or tunnel. For example, some oblique crossings use a more orthogonal structure where the road crossing occupies only part of the surface of the structure; or where there are buildings on the bridge perhaps so the bridge is wider than the road (if any) it supports.

Tags

Key	Value	Discussion
type	bridge / tunnel
layer	layer number	If omitted, should be assumed 1 for a bridge and -1 for a tunnel; sometimes a bridge crosses a bridge so layer=2 might be needed.
name	a name	The bridge or tunnel is known by this name (e.g. "Tower Bridge")
ref	a reference	Bridges are sometimes numbered (e.g. along UK canals)
toll	yes	it is a toll bridge/tunnel (but does this make sense for the relation? What if it's a free bridge over a tolled canal?) This is a feature of the footway, the cycleway and the motorway through a tunnel and over a bridge and can be different!
height	height in meters	Bridge height measured from ground to top (usually derived from a signpost on the bridge or its side)
length	length in meters
maxheight	height in meters	the maximum permissible height for bridges/tunnels with limited clearance (usually derived from a signpost on the bridge) (as per existing tag in Map_features) This is a feature of the footway, the cycleway and the motorway through a tunnel and over a bridge and can be different!
maxweight	weight in tonnes	the maximum permissible weight for a vehicle passing over the bridge (usually derived from a signpost on the bridge) (as per existing tag in Map_features) This is a feature of the footway, the cycleway and the motorway through a tunnel and over a bridge and can be different!
maxwidth	width in meters	the maximum permissible width for a vehicle passing over the bridge/in the tunnel (usually derived from a signpost on the bridge) (as per existing tag in Map_features)This is a feature of the footway, the cycleway and the motorway through a tunnel and over a bridge and can be different!
operator	operator name	The party responsible for the bridge, e.g. "Network Rail"

Members

Way or Node	Role	Recurrence?	Discussion
		zero or more	the ways supported by (on top of) a bridge / passing through a tunnel
	under	zero or more (bridge)	the ways passing under a bridge
	outline	optionally one	a way forming the outline "footprint" of a bridge or tunnel
	edge	zero or more	alternative to outline, a set of ways which form the edges. Rather than drawing an area, a renderer could then draw the parapet marks as now along these ways. The interior of the bridge should be on the right of these ways (so if we were modelling a simple bridge using this technique, there would be two ways running in opposite directions parallel to and either side of the road-way which passes across the bridge; but this is generalisable to more complex cases: for example, consider a bridge which has a meeting of three ways on top). If neither outline nor edge is given, the "across"/"through" ways would be used to derive a nominal outline for rendering purposes for example.

Discussion

Please discuss in the talk-section