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.
Ramps
Ramps have a very specific purpose in Waze. They are intended to connect segments of Minor Highways, Major Highways, and Freeways to roads where there are no at-grade crossings.
When to use ramps
Use of the Ramp type is governed by the following rules:
How complex should ramps be?
Rule #1 is still simpler is better. If there is no large distance between paths at the end of a ramp (either into or out of the ramp), a single segment connecting to a single junction node is all that is needed. The existence of a painted, concrete, or grass island is NOT enough of a reason to split a ramp into multiple ramps.
When paths at the end of the ramp deviate significantly in distance, regardless of the existence of any type of island, then multiple ramps should be used.
Highway/Freeway Junctions
These are junctions involving the three Highway/Freeway road types - Minor Highway , Major Highway , and Freeway -- as well as their Ramps .
Specific examples of how to handle common junction types are provided in later sections. All of those examples use the basic building blocks provided here.
If you are unsure what road type you should use, refer to the following pages for more information.
Highway/Freeway Exits
It is a basic Exit situation when a "straight" direction is obvious to a driver and navigation instructions are only needed for the non-straight direction (the exit.) If navigation instructions are required for both directions, see the Wayfinder Segments section below.
Structure
To be treated as a basic Exit, the following must be true:
- The entering segment and one exiting segment must be one of the three Highway/Freeway types
- The Highway/Freeway exiting segment must have close to a zero degree departure angle from the entering segment
- The other exiting segment must be of the type Ramp
- The Ramp exiting segment must have a departure angle of between 20 and 30 degrees from the entering segment
When those conditions are met, the navigation will present an "Exit Right/Left" instruction when the ramp is to be used, and will remain silent when the exiting Highway/Freeway segment is to be used.
Naming
The Highway/Freeway segments before and after the junction should be named the same. The ramp segments should be named in accordance with the best practices in your location.
Naming standards for other areas
Editors covering areas that do not have specific best practices should review the existing guides for other areas, and determine which best matches the roadways of your area.
Highway/Freeway Splits
A Highway/Freeway Split is when a Highway/Freeway segment meets at a junction with two other Highway/Freeway segments and there is no obvious straight through direction to a driver.
Structure
To receive a navigation instruction for both branches of a split, the following must be true:
- All segments must be one of the three Highway/Freeway Types
- All segments must have names which are different from each other
- The two exiting segments must have departure angles of 20 to 30 degrees from the entering segment
With those conditions met, the junction will present "Keep Left" and "Keep Right" navigation instructions using the name of the appropriate exiting segment.
Naming
The primary rule is that all 3 segments at the junction must have different names. That can be accomplished in one of two ways:
- Using road names alone - It is an easy situation if all three roads which connect have different names. If "Highway A" splits into "Highway B" and "Highway C", then that is all we need to have a properly functioning split.
- Using signs and Wayfinder Segments - If one of the branches of the split has the same name as the entering segment, we must create uniqueness at the junction. If "Highway X" splits off from "Highway Y" and "Highway X" continues as the other branch, the preferred approach is to use named Wayfinder Segments.
Wayfinder Segments
It may be necessary to provide additional information to a driver for complex or confusing Exits and Splits. Examples of such situations include:
- Lane Drops - Highway has been 3 lanes for miles and miles but only 2 lanes continue straight through at a certain point
- Inconsistent signage - Highway continues as a numbered route, but signs only call it by a name instead
- Non-obvious continuations - in a right hand drive country, exiting traffic is to the left and continuing traffic is to the right
In these cases we need to use short way-finder or path-finder segments which are named with the information displayed on the roadway signs. There are two methods to accomplish this: using Highway/Freeways or using Ramps.
- Highway/Freeways
- Pro: provide a consistently rendered line on the map with no breaks
- Pro: does not introduce a routing penalty for the transition to Ramp type (although we do not know how much of an impact, if any, this has in actuality)
- Con: the long name of the segment may be displayed on the map creating clutter
- Con: the segment may be hard to see in the editor since it may blend in with the main Freeway
- Note: will provide "Keep Left" and "Keep Right" instructions
- Ramps
- Pro: ensures any long names are hidden from display on the map
- Pro: forces the client to stay zoomed in for the length of the segment to give a close view of the split to the driver
- Con: introduces a routing penalty of the transition to Ramp type (although we do not know how much of an impact, if any, this has in actuality)
- Con: may render as a broken line on the map
- Note: will provide "Keep Left" and "Exit Right" instructions in right-hand drive countries and "Exit Left" and "Keep Right" instructions in left-hand drive countries.
In both cases, the two exiting segments MUST have identical road types and different names from themselves and the entering segment.
Here we show Highway Y splitting off from Highway X. By labeling segments with the information available on the road signs at the split, we have achieved uniqueness and provided additional useful information (the destination cities) to the driver. We can achieve the desired results using either approach. First as all Freeway segments:
And then using Ramps (which are named how they appear in the all Freeway example):
At times it is also necessary to use a way-finder at an Exit if a driver needs advance notice that only some lanes of the roadway continue straight through. Again we can accomplish this using either of the two methods. First using Freeway segments for the split (the right branch wayfinder is un-named and the ramp segment that follows contains the appropriate name):
And then using Ramps (again, named the same as above):
A rule of thumb for the way-finder segments is to make them 15 to 20 meters long. This keeps the segment small so we are less likely to see it in the client, but keeps it long enough to find and manipulate in the map editor. (In the near future, Way-finder segments may need to be greater than 5 meters long to prevent routing issues.)
Ramp to Ramp Splits
A ramp may itself split and branch into two directions. If this is the case, "Exit Right" and "Exit Left" will be announced using the name of the appropriate exiting segment in all cases.
Structure
Naming
If ramps are unnamed, the name of a subsequent ramp will propagate backwards. In the example above, if the two ramps exiting the junction are named, the ramp entering the junction can be left unnamed. Then any navigation instruction directing you onto the first ramp would use the name of the appropriate exiting ramp.
Example: The two ramps exiting the junction are named "DestinationLeft" and "DestinationRight". The ramp that enters the junction is unnamed. If you need to "Exit Right" onto the unnamed ramp. If you are headed to "DestinationLeft", navigation would tell you:
- Exit Right to Destination Left
- Exit Left to Destination Left
Using unnamed ramps is very useful to provide sufficient notification of an approaching decision point, as long as the names of both ramp splits are visible on signs at the start of the initial ramp.
Example of good use of unnamed ramps:
- Initial Exit Sign: to City A and City B
- Destination Left Sign: to City A
- Destination Right Sign: to City B
Result: An unnamed initial ramp will provide accurate and informative navigation instructions to the driver.
Example of poor use of unnamed ramps:
- Initial Exit Sign: to Downtown
- Destination Left Sign: to Downtown
- Destination Right Sign: to Center St
Result: An unnamed ramp may create confusion since both Destination ramp names are NOT listed on the initial exit sign. In this case the initial ramp should be named.
Example of modified use of unnamed ramps:
- Initial Exit Sign: Exit 70A-B to City A and City B
- Destination Left Sign: to City A
- Destination Right Sign: to City B
- Destination Left name in Waze: Exit 70A: City A
- Destination Right name in Waze: Exit 70B: City B
Result: By using a modified name for the destination ramps, we have combined information from two sets of signs to generate the advance notice a driver may need to prepare for a decision point.
Limited Access Interchanges
Please see the Limited Access Interchange Style Guide.
Diamond Interchange
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.
Cloverleaf Interchange
See also: Cloverleaf Interchange article on Wikipedia
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.
Folded Diamond
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.
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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)
See also: Single Point Urban Interchange article on Wikipedia
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.
Collector/Distributor Lanes
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/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.
See also
Review the Wikipedia article on Road Interchanges for further information on this topic.