The two surface street segments (between the outer ramps and connected to the Single Point) and the 4 ramp segments connected to the single point should all be the same level, either one higher or one lower than the elevation of the freeway segments above/below the single point.
These are lanes parallel to but physically separated from the lanes of a
Limited Access Road that serve to keep merging traffic out of the flow of through traffic on the mainline Freeway.
==== Collector/distributor cloverleaf ====
This is a Cloverleaf interchange that is connected to a Collector/Distributor instead of directly to the main roadway. Here is an example which matches the physical world but has a major deficiency.
At first it appears that this layout allows everyone to get where they are going. The problem is that it allows MORE than it should. It is possible to exit the mainline Freeway and stay on the Collector/Distributor, bypassing the Cloverleaf, and merge back onto the mainline Freeway.
Although this may be a way to avoid traffic on the mainline Freeway, doing so is inefficient, may be unsafe, and is outright illegal in some areas. Therefore we need to tweak our design a little...
In this example, we have eliminated the ramp segment joining the two loops of the cloverleaf and the Collector/Distributor ramps. Now they all connect at a single point and we are therefore able to restrict the turn that allowed drivers to use the Collector/Distributor as a bypass. ==== Complex collector/distributor ==== A Collector/Distributor is considered complex if there are multiple exits and entrances connected to the Collector/Distributor.
Ideally you will '''not''' have to implement any of the complex layouts described in this section. Start by representing the ramps in the simplest manner possible and see how they perform for a while. If everything is mapped correctly and navigation still tries to have drivers bypass the mainline Freeway, then and only then modify the layout with the following suggestions.
If we are lucky, we can still restrict the bypass movement with the single central node like we did for the [[#Collector.2FDistributor_Cloverleaf|Collector/ Distributor Cloverleaf]] example. Notice how all the exits are either before or at the central node (highlighted in the next image) and all the entrances are at or after the central node.
It becomes more complicated if entrances and exits are more mixed. In the following example, there is an entrance before an exit and the highlighted path shows this entering traffic crossing the exiting traffic we dealt with in previous examples. If we use the same restricted turn on the same node as in previous examples, we block traffic entering from the right-most entrance from being able to get to the mainline Freeway. If we enable that turn, now we are no longer blocking the bypass movement.
To get the control we require, we need to run the entrance ramp up to the node of the last exit. This allows the exiting traffic to be kept separate from the entering traffic. In the next example, we have modified the right- most entrance so it connects further down the Collector/Distributor. Even if the Collector/Distributor itself is only one lane, we want to have these parallel road segments through the area. They can be close together, but do not overlap them. You will have segments overlapping, but be sure not to junction them together.
Now we can restrict the exiting traffic from using the Collector/ Distributor as a bypass of the mainline Freeway as we have previously without impacting the entering traffic.
In a more complicated case, you will need to create parallel paths for exiting and entering traffic for a majority of the Collector/ Distributor's length. In the following example, the path for exiting traffic is highlighted and the ramps available for entering traffic appear normally. There is no mixing of entering and exiting traffic in this logical view of the Collector/Distributor even if traffic is mixing in reality.
Image: Jct_cmplx_collector_3. png]]
As a worse case scenario, you may have a situation where traffic can enter early in the Collector/Distributor and can legally exit at any of the exits along the Collector/Distributor. In this case you will need to carefully create junctions with restricted turns or even additional ramps to enable the legal movements.
== See also ==
Review the [http://en.wikipedia.org/wiki/Interchange_(road) Wikipedia article on Road Interchanges] for further information on this topic.