Limited Access Interchange Style Guide
When two roads meet in a limited access interchange (any situation where travel between grade separated roads is facilitated by ramps or slip roads alone), extra care must be taken.
If an exit ramp lines up with an entrance ramp, care must be taken to restrict the straight through direction to prevent "ramp-to-ramp routing" where the routing engine may try to take a shortcut from the Freeway, to the exit ramp, to the entrance ramp, and back to the Freeway instead of staying on the Freeway. That motion is inefficient and may be illegal in certain jurisdictions.
Care must be taken to not enable all turns in any situation where ramps meet unless all possible flows of traffic have been considered!
We also have to pay attention to the elevation of the road segments since there will typically be many bridges and overpasses for any limited access interchange. If two roads cross without connecting, their elevations must be different.
The following sections discuss some of the common Interchange designs. Note that some Interchanges may be a hybrid of these basic designs where one side or quadrant of the interchange may differ from the others.
Before reading through this article, be sure to fully understand the information in the Junction Style Guide.
See also: Diamond Interchange article on Wikipedia
Common in wide open spaces where land acquisition and geography are not concerns, this Interchange design has ramps equally distributed across all 4 quadrants.
In the simplest form, this can be represented as single connections from the ramps to the surface street.
Note: Be sure to restrict the straight through motion from the exit ramp onto the entrance ramp on the other side of the road. This will prevent the routing server from trying to route someone off the freeway just to get back on it. Even though it may be a legal direction for a vehicle, turn restrictions are only for controlling routing directions.
If the ramps connect to the surface street at multiple points, ramp-to-ramp routing should be avoided as well as illegal turns which should use another ramp. Review the section How complex should ramps be? in the Junction Style Guide for more details on this topic.
First we see the turns that must be restricted for the exit ramps:
Then we see what must be restricted for the entrance ramps:
Note on elevations: The single surface street segment between the inner most ramps should be either raised or lowered in relation to the freeway segments depending on the actual geography at the interchange.
In a Cloverleaf Interchange, left turns are eliminated from all movements between the Freeway and the surface street. First check the exit ramps.
Then check the entrance ramps for illegal turns.
The connections to the Freeway segments may be treated in two ways:
- (top) we can have the inner entrance and exit ramps have their own junction nodes with the Freeway. Do NOT use this approach if there are Collector/Distributor Lanes (or a similar situation) involved.
- (bottom) we can have the entrance and exit ramps share a single junction node with the Freeway. This allows us to eliminate the very short Freeway segment that may exist between the inner entrance and exit ramps.
It is best to offset this shared junction onto the Entrance ramp side of the surface street. This prevents the junction from accidentally being connected to the surface street or looking like it does. We favor the Entrance ramp side, because this would result in a slightly earlier exit instruction which is of course preferred over a late exit instruction.
The determining factor of which design to use will partly depend on the actual size and scale of the specific interchange and if there is a Collector/Distributor involved.
Note on Elevation: The single surface street segment between the inner most ramps should be either raised or lowered in relation to the freeway segments depending on the actual geography at the interchange.
See also: Discussion of Folded Diamonds and A2/B2 Partial Cloverleafs on the Partial Cloverleaf Interchange article on Wikipedia
Geography or property ownership may prevent the ability for an interchange to be constructed with all ramps evenly distributed across the 4 quadrants of the interchange. When only two quadrants are used, it is typically called a Folded Diamond (basically a sub-type of a Partial Cloverleaf Interchange). The ramps may be all on one side (as in the examples in this section) or they may be located in diagonally opposed quadrants.
The unique situation presented by the Folded Diamond arrangement is having both Entrance and Exit ramps terminating on the same side of the surface street. Ideally both ramps should terminate on the same junction node to permit us to easily restrict the illegal and usually impossible ramp-to-ramp movement.
Like with a basic Diamond Interchange, often it will be necessary to represent the ramps making multiple connections to the surface street. Be sure to read the article How complex should ramps be? in the Junction Style Guide.
Restrict all non-permitted turns.
Note on Elevation: Similar to a basic Diamond interchange, in most cases only the segment of the surface street that crosses the Freeway segments will need to be adjusted up or down.
Single Point Urban Interchange (SPUI)
A SPUI is a very space and flow efficient design, but it takes extra attention to ensure the turns are correct. And as the name indicates, ideally there should be a single junction in the center. You may need to tweak the geometry of segments a bit off of alignment from the real physical world, but it should be minor if the interchange is a true SPUI.
The outer branches of the exit ramps are very much like in the case of a diamond interchange:
Where things get complicated is the inner branches leading to the Single Point. You need to avoid ramp-to-ramp in two directions and a reverse flow turn. Note: The ramp-to-ramp motion to facilitate a U-Turn (the top left arrow in the image below) may or may not be allowed depending on the specific interchange. Please validate this turn.
Luckily the entrance ramp restrictions are similar to the diamond interchange:
If you were to look at all the restricted turns at once, you may get the false impression that something is very wrong. But as you now know, a SPUI has almost as many restricted turns as allowed ones.
Note on Elevation: 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.
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.
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/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.
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.
Review the Wikipedia article on Road Interchanges for further information on this topic.