Key:maxspeed:practical
 maxspeed:practical
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Realistic speed estimate in cases where official speed limits are meaningless and speed information can not be estimated in other ways. 
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| Group: restrictions |
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Status: in use
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Map realistic average speed estimate in cases where there are no legal speed limits or the realistic traveling speed is much lower than the posted maximum or advisory speed and realistic speed information can not be estimated in other ways.
Good examples are narrow winding mountain or rural roads or desert tracks that do not have posted speed limits.
This key should not serve as an example for the introduction of similar keys. It is used for lack of a better alternative such as live traffic data. Some mappers object that the use of this key violates the principle of verifiability. The original proposal for this tag was rejected - see Proposed features/Practical maxspeed.
How to map
maxspeed:practical can be used to map a "realistic average speed" on highways where other tags are not sufficient to describe what kind of traveling speed could be reasonably expected.
Examples:
- maxspeed:practical=15 - you can expect to drive around 15km/h
Use of maxspeed:practical > maxspeed is discouraged and routers are expected to ignore such values.
Where particular obstacles (such as traffic lights, ferries) can be mapped appropriately by other tags they should be mapped and maxspeed:practical should be applied to the segments between the obstacles so that the total driving speed can be approximately estimated by combining the information about mapped obstacles and maxspeed:practical. From this follows that maxspeed:practical is easiest to apply to long extra urban stretches of road where such obstacles play only a minor role. In urban environments it can be applied where some roads are particularly slow due to congestion or other factors.
Use with conditions
Conditional restrictions could be used such as:
- maxspeed:practical:conditional=30 @ (07:00-09:00); 40 @ (16:00-19:00) + maxspeed:practical=60
- maxspeed:practical:hgv:conditional=20 @ (07:00-09:00); 30 @ (16:00-19:00) + maxspeed:practical:hgv=50
- maxspeed:practical:conditional=30 @ wet - expect an average speed of 30 when wet
Data consumers may not support this for some time so it is important to specify a fallback.
Expected evaluation by routing software is to apply the most specific matching value of maxspeed:practical and if none matches fall back to the most specific values of maxspeed:advisory=* and maxspeed=* as described here: Conditional restrictions#Evaluation of conflicting restrictions. Hence there is no use to specify maxspeed:practical in conditions where maxspeed or maxspeed:advisory are sufficient.
Old style conditions
This syntax was invented before the approval of Conditional restrictions and in theory may be still evaluated by some data consumers though none is known.
- maxspeed:practical=90;nighttime:70 - you can expect to drive around 90 km/h at daytime, 70 km/h during night hours.
- maxspeed:practical=60;rushhour:20 - 60 km/h at all times except rush hour.
See also
- maxspeed=*
- maxspeed:advisory=*
- Conditional restrictions - should use this notation as far as possible
- Routing/Travel Time Analysis - various possibilities to analyse GPX tracks to get average speed and delays per segment
- Average speed per way - other thoughts how it could be done, unfortunately defunct
- Proposed features/traffic speed - fairly similar proposal
- Global Statistical Speed Matrix - another work in progress concept of speed data storage
History
This key was originally introduced by the rejected proposal Proposed features/Practical maxspeed nevertheless remained in use ever since.
Current description and use differ substantially from that of the rejected proposal.