References:
* http://wiki.openstreetmap.org/wiki/Access
* http://wiki.openstreetmap.org/wiki/OSM_tags_for_routing/Access-Restrictions
* http://wiki.openstreetmap.org/wiki/Conditional_restrictions

Supported vehicles:
* foot (foot=*)
* bicycle (bicycle=*)
* car (motor_vehicle=*)

Access must be separatly evaluated for each vehicle type and for both
directions.

Speed restrictions may also depend on vehicle type.

Tag to evaluate:
* access(:backward/foward)=true => foot, bicycle, car
* (access:)foot(:backward/forward) => foot
* (access:)bicylce(:backward/forward) => bicycle
* (access:)motor_vehicle(:backward/forward) => car
* (access:)motorcar(:backward/forward) => car
* oneway => bicycle, car
