GraphHopper supports turn restrictions. Turn restrictions are crucial for correct vehicle navigation and help to avoid forbidden turns.
Without turn restrictions (the turn is not allowed):
Turn restrictions have to be enabled on a vehicle basis. To enable it for one vehicle add
|turn_costs=true in the config, for example: graph.vehicles=car|turn_costs=true.
Or when using the Java API directly you can either create the encoding manager like EncodingManager.create("car|turn_costs=true") or
new EncodingManager.Builder().add(new CarFlagEncoder(5, 5, 1) where the last parameter of CarFlagEncoder represents
the maximum turn costs (a value of 1 means the turn can either be legal or forbidden).
Turn restrictions are not available for every vehicle as they have low relevance
for some vehicles like foot.
To enable turn restrictions when using the 'speed mode' additional graph preparation is required, because turn restrictions
require edge-based (vs. node-based) traversal of the graph. You have to configure the profiles for which the graph
preparation should be run using e.g. profiles_ch, just like when you use the 'speed mode' without turn restrictions.
The edge-based CH preparation will be chosen if you configure turn_costs: true for the profile you are referencing
in profiles_ch.
You can also specify a time penalty for taking u-turns (turning from one road back to the same road at a junction).
Note that this time-penalty only works reasonably when your weighting is time-based (like "fastest"). To use u-turn
costs with speed mode you need to specify the time penalty for each u-turn (again in the profile configuration):
u_turn_costs: 60. See config-example.yml for further details regarding these configurations.
If you prepare multiple 'speed mode' profiles you have to specify which
one to use at request time: Use the edge_based=true/false parameter to enforce edge-based or node-based routing and
the u_turn_costs parameter to specify the u-turn costs (only needed if there are multiple edge-based 'speed mode'
profiles with different u-turn costs). To disable the 'speed mode' per request you can add ch.disable=true and choose
the value of u_turn_costs freely.
While OSM data only contains turn restrictions, the GraphHopper routing engine can also deal with turn costs, i.e. you can specify custom turn costs for each turn at each junction.
Conditional turn restriction are supported. For example, the following no left turn restriction concerns only bus :
type=restriction
restriction:bus=no_left_turn
Another example, using the except tag, means only bicycle are allowed to turn left:
type=restriction
restriction=no_left_turn
except=bicycle
You can overwrite FlagEncoder#acceptsTurnRelation to change the default handling of turn restrictions in your customized vehicle profile.