U-turn policy question

thomasbales

I am trying to model u-turns on a railway. I know how to do this by using u-turn restrictions at each node. However, this is going to be a superman type feat and it would be much easier if I could turn off u-turns altogether and model a feature to allow a u-turn instead of restrict it. Is there a way to do this in a network dataset?

PatrickStevens

Hi, Thomas!  I've been discussing this issue with colleagues.  We came up with a solution we think might work.

1.  Make all of the junctions you want to ALLOW u-turns as user-defined junctions.

You can select that set of system junctions, export it as a feature class in the same feature dataset as your network, then add it as a source to your network.

2. Add a descriptor attribute to your network called something like uturn_restricted_at_junction.

     a. Set the descriptor to boolean

     b. Set the default junction evaluator value to a constant of true

     c. Set the source junction evaluator value for your user junctions to a constant of false

3. Make a restriction attribute on your network called something like restricted_uturns

     a. Set the type to boolean

     b. On the default evaluator for turns, set it to script and add this python script:

In the pre-logic script code, put:

def SetValue():

  is_restricted = False

  if fromEdge.EID == toEdge.EID:

    is_restricted = atJunction.AttributeValueByName( "uturn_restricted_at_junction" )

  return is_restricted

if the value = part, put:

SetValue()

4. Make sure you already have a turn feature class in your network.  For my toy network, I added an empty one.  Seems like a turn source is required to get this to work, even if there is nothing in the source feature class.

Then, test that it works!  I tried this on a toy network and it worked.  Uturns at my user junctions were allowed and uturns at my system junctions were not allowed.

You can test it by adding a stop near a test junction on the right side on the edge.  Set the curbapproach for the stop to right. 

Then, add a second stop farther away from the test junction, so that the route has to leave stop1 towards the junction and the quickest route would be a uturn at the test junction back to stop2.

I hope all of this makes sense.  Good luck!