Handling of generic access tags

[mattwigway]

In the car profile, access=yes is currently treated as allowing traversal regardless of path type - for instance this corridor is considered accessible to cars by OSRM. This interpretation follows the letter of the definition of the access tag, but perhaps not the spirit; I suspect the vast majority of ways tagged this way are not actually accessible to motor vehicles. From the OSM wiki:

In theory, adding access=yes to highway=footway could be read as changing default restrictions (which usually are foot=yes and vehicle=no for highway=footway) to yes, highway=footway + access=yes means "road, which is open for all pedestrians and vehicles".

In practice, this combination is often used by mappers to modify (rather than enlarge) default values: for example, access=permissive with highway=steps is very unlikely to be traversable by a truck, whatever the tags may say.

Maybe we should only allow traversal by motor vehicles on ways that generally wouldn't allow them (e.g. footway, cycleway, steps, elevator, corridor) if access:motor_vehicle or access:motorcar is set?