How To: Use U-turn features as a restriction in solving Network Analyst layers
A turn feature class may be used to model the restriction of certain types of turns. In some cases there may be intersections or junctions where a U-turn is not allowed. A turn feature class containing U-turn features can be used to restrict turns made at these locations. It is important to note that simply having a turn feature class enabled as a restriction will not guarantee that U-turn restrictions will be honored. There are other settings such as curb approach and side of edge that are important.
This article explains how to correctly model U-turn restrictions given an already created turn feature class containing turn and U-turn features.
Once turn features and U-turn features have been created in a turn feature class associated with a Network Dataset, an attribute restriction can be created using the turn feature class. The following screen shot shows how the restriction is set up in the Network Dataset properties.
U-turn features should look like the following in a network dataset.
Once this U-turn feature is created and a turn attribute restriction is in place, modeling U-turn restrictions can take place.
The first step in modeling U-turn restrictions in a network analysis layers is understanding curb approach and side of edge settings. If a vehicle is able to reach a stop or location from either side of the vehicle then it may not be necessary to make a U-turn. This would cause the U-turn restriction to be disregarded all together.
In the following example the final stop is on the left side of the road, but since the vehicle can make a stop on either side there is no need for a U-turn. The vehicle in this scenario can simply pull over to the left and reach the stop.
In most routing scenarios a vehicle needs to make a curb approach from the right side of the vehicle. This means in order to visit stop on the left side of the street the vehicle will need to make a U-turn somewhere or circle a block. By using the layer properties of the route layer, it can be specified that the curb approach for all stops is on the right side of the vehicle.
With the turn restrictions in place, and curb approach set to right side of vehicle, U-turn restrictions are correctly modeled in the route. In this example the route extends past the restricted U-turn feature and goes to the next intersection with permissible U-turns. The route then continues back to the final stop reaching it on the right side of the vehicle.
When loading points to a route layer or any network analyst layer it is important that they have the correct curb approach assigned and they are located on the correct side of the edge. Otherwise U-turn restrictions will not be enforced correctly.