|Sidewalks (also footways or pavements) are provided beside the carriageway of a highway for use by pedestrians and sometimes also cyclists.|
The sidewalk (or pavement) is that part of a highway set aside for the use of pedestrians, often separated from the carriageway (or roadway) by a kerb (also curb). A sidewalk may be separated from the carriageway by only a kerb, by a road verge or alternatively may be at some distance from the road (but still associated with it) or separated from the road by some form of barrier, for example bushes or a line of trees. A road may have a sidewalk on only one side of the carriageway, or both side or have no sidewalks. highway=living_streets don't have sidewalks, but are accessible to pedestrians.
Caution over use of terms:
The legal UK term for a pavement or a sidewalk is footway. However, the term footway is used within OSM for any path, be it beside a carriageway or otherwise. The term pavement which is often used in the UK as an alternative to footway is used in the USA for the surface of the carriageway (which incidentally is known as a roadway in the USA!)
So because footway and pavement, the British English alternatives, are open to confusion, the North American English sidewalk is used in this case even though British English is the preferred language for OSM tagging and wiki.
How to map
The inclusion on sidewalk information makes it easier to provide effective pedestrian routing, and in particular good narrative descriptions of pedestrian routes along motorised roads. The sidewalk tag is not needed on non-motorised thoroughfares, for example highway=footway/cycleway/path/brideway/track.
Sidewalk as refinement to a highway
The simplest method is to tag the associated highway with sidewalk=both/left/right/none as appropriate for those sections of sidewalk that are parallel with the carriageway and to use highway=footway where the path diverges from the carriageway.
There is not yet an established approach for adding attributes to sidewalks tagged using this method. Suggestions include the currently abandoned Proposed features/Advanced footway and cycleway proposal.
Sidewalk as separate way
A different approach is to map each sidewalk as separate ways using highway=footway and an associated footway=sidewalk with footway=crossing used for places where the footway crosses a street. This method allows for a more spatially accurate representation of the pedestrian environment and allows a more straightforward use of barrier=*, tactile_paving=*, kerb=*. (In comparison, a minor issue with the method above is that where a pedestrian route diverges from highway at an angle it will currently be mislocated due to the width of the rendered road or has a small part at a wrong angle.) The method produces a visible result in existing routing engines and renderers, as they will usually not understand the footway=sidewalk tag and treat it the same manner as footways that are not associated with a road. For this reason, this approach tends to produce a more cluttered map and occasionally other undesirable effects. Future pedestrian routers would need to associate the footway way with the highway to produce narrative directions (e.g. "Follow the sidewalk along Main Street") and to allow crossing smaller roads at any point along the road, which will require grouping the separate ways by e.g. using a relation or surrounding both ways with a area:highway=* area.
Access information for pedestrians
The above tagging provides information about the physical infrastructure rather than legal access details which can be included using foot=yes/no. The assumption is that all roads are legally accessible to pedestrians except for highway=motorway where it is assumed that pedestrians are not allowed unless otherwise stated.
A view of sidewalks in Washington from ITO Map showing roads with sidewalks in dark green, those without sidewalks in dark red and other paths in light green. A mapCSS stylesheet is available for josm.