Compulsory use of parallel way
The access=* value use_sidepath is used to indicate that specific classes of transport modes must use a mapped parallel way instead. Typically, this access value is used with:
The use_sidepath access value should not be used with general the top-level access key (which is used for all modes of transportation); it always applies to a specific mode of transportation that is normally allowed access on the tagged way, but traffic law applied to the local circumstances mandate the use of a parallel way (such as a signposted cycleway, footway or bridleway) instead.
Legal basis
For countries in which the legislation follows the Vienna Convention on Road Signs and Signals (contracting parties as well as other countries that voluntarily have implemented these provisions) this mandatory use has its basis in the following provisions :
Section D MANDATORY SIGNS
4. Compulsory cycle track Sign D, 4 "COMPULSORY CYCLE TRACK" shall notify cyclists that the track at the entrance to which it is placed is reserved for them, and shall notify the drivers of other vehicles that they are not entitled to use that track. Cyclists shall be required to use the track if the track is running along a carriageway, footpath or track for riders on horseback and leading into the same direction. However, drivers of mopeds shall also be required, in the same conditions, to use the cycle track if domestic legislation so provides or if notification of this requirement is given by an additional panel bearing an inscription or the symbol of sign C, 3d.
Sign D, 5 "COMPULSORY FOOTPATH" shall notify pedestrians that the path at the entrance to which it is placed is reserved for them, and shall notify other road users that they are not entitled to use that path. Pedestrians shall be required to use the path if the path is running along a carriageway, cycle track or track for riders on horseback and leading into the same direction.
6. Compulsory track for riders on horseback Sign D, 6 "COMPULSORY TRACK FOR RIDERS ON HORSEBACK" shall notify riders on horseback that the track at the entrance of which it is placed is reserved for them and shall notify other road users that they are not entitled to use that track. Riders on horseback shall be required to use the track if the track is running along a carriageway, cycle track or footpath and leading into the same direction.
These provisions apply when formal traffic signs are placed on the track, such as 
However, when other types of signs (non-formal traffic signs) are applied instead to inform that use of the track is allowed for these specific road users, this usage may not be mandatory (the carriageway might be used instead), and may not be reserved for these road users only (pedestrians and cyclists might be allowed on a "non formal" bridleway), depending on the legislation and local access signs.