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How To: Use U-turn features as a restriction in solving Network Analyst layers

Summary

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.

Creating a turn feature that represents a U-turn

Procedure

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.
[O-Image] Turn restriction setup

  1. U-turn features should look like the following in a network dataset.
    [O-Image] U-turn feature
    Once this U-turn feature is created and a turn attribute restriction is in place, modeling U-turn restrictions can take place.

  2. 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.
    [O-Image] Curb approach either side
  3. 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.
    [O-Image] Curb Approach Layer Properties
  4. 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.
    [O-Image] right side of 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.

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