An interchange is a road junction where two roads are connected by dedicated roadways, called ramps. The roads connected by an interchange do not intersect one another directly, and if they cross, the crossing is grade-separated.
Since interchanges often involve grade-separated crossings, the road elevation of the segments becomes important. If two roads cross without connecting directly, their elevations must be different.
This article is a sub-article of the Junction Style Guide. As such, this article is a style guide as well. The following sections discuss the proper style for ramps, interchanges, and some 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.
The Ramp type is used extensively in interchanges for three reasons.
- Ramp segment names are not displayed on the map.
- Ramp segments have essentially no penalty, so they can be used to connect freeways and major highways with each other without causing problems.
- Ramp segments are relatively small but show at high zoom levels, so interchanges do not distract from highways but can be seen at high speeds.
When to use ramps
Use of the Ramp type is governed by the following rules:
Ramp geometry and complexity
When placing the beginning of a ramp, place the first geometry node of the ramp segment at the point where the solid white line begins, and extend the ramp naturally to create a junction with the road segment, typically at a 10–15° angle. This will allow for consistent timing of exit instructions.
Do the same when placing the end of a ramp. This will allow for consistent auto-zoom functionality—the client remains zoomed in for the duration of the ramp, zooming out when the user gets on the freeway.
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 divide 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.
Component junction types
Interchanges are made up of multiple individual junctions involving the following:
- One or more highways, represented by the three highway road types Minor Highway , Major Highway , and Freeway
- Grade-separated cross streets which may be highways or lower road types such as Primary Street
- Ramps to join them
Specific examples of how to handle common junction types are provided in the following sections. All of those examples use these road types. If you are unsure what road type you should use, refer to the road types article.
Basic exit
A basic exit exists 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 wayfinders section below.
Basic exit geometry
To be treated as a basic exit, the following must be true:
- The entering segment and the continuing segment must be Freeway , Major Highway , or Minor Highway type,
- The freeway/Highway continuing segment should have close to a zero degree departure angle from the entering segment,
- The other exiting segment must be of the type Ramp , and
- The Ramp exiting segment should have a departure angle around 10–15 degrees from the entering segment, as explained above.
When those conditions are met and the ramp exits to the right, the navigation will present an "exit right" instruction when the ramp is to be used, and will remain silent when the continuing freeway/highway segment is to be used. When those conditions are met and the ramps exits to the left, the default instruction in the USA is "stay to the left." A turn instruction override should be used to provide an "exit left" instruction at basic exits where the exit ramp departs the highway to the left.
Basic exit naming
The highway/freeway segments before and after the junction should be named the same. The ramp segments should be named in accordance with this section on ramp names.
Complex exit
Freeway/highway fork
A highway/freeway fork 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. This is synonymous with the MUTCD term "split".
Freeway fork geometry
To create a default navigation instruction for both branches of a fork, the following must be true:
- Both exiting segments must be the same type (freeway, major highway, minor highway).
- Each exiting segment must have a name which is different from the name of the entering segment.
- The two exiting segments should have departure angles around 10 to 15 degrees from the entering segment.
With those conditions met, the junction will present "stay to the left" and "stay to the right" navigation instructions using the name of the appropriate exiting segment.
Freeway fork 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" forks into "Highway B" and "Highway C", then that is all we need to have a properly functioning fork.
- Using signs and wayfinder segments - If one of the branches of the fork has the same name as the entering segment, we must create uniqueness at the junction. If the outgoing segment has a big green sign (BGS) above it that says something different than the name of the entering segment, the preferred approach is to use named wayfinder segments.
Freeway fork instruction override
If one branch is named the same as the entering segment, and there is no sign above it that says anything different from the entering segment, use a turn instruction override to override the default behavior and give a "stay to the left" or "stay to the right" instruction depending upon which direction the branch leaves the fork.
Ramp fork
A ramp may itself fork 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.
Ramp fork geometry
Ramp fork 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
- Keep 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 forks 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.
Configurations
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.
The straight through motion from the exit ramp to the entrance ramp should typically be enabled, if legal to drive. Under normal circumstances, the Big Detour Prevention mechanism discourages the routing server from routing someone off the freeway and directly back on. When the freeway path between the ramps is closed, or slow enough to overcome the Detour penalty, this off-on route may be given as a desirable alternative.
Be aware that the Big Detour Prevention penalty is intended to discourage routing that leaves a freeway (or highway) and returns to the same freeway (or highway). Therefore, at least one name (primary or alternate) of the freeway/highway segment before the exit ramp must exactly match one name (primary or alternate) of the freeway/highway segment after the entrance ramp to trigger the penalty. For further information see the Big Detour Prevention Mechanism page.
If the ramps connect to the surface street at multiple points, restrict turns which should use another ramp. Review the section on ramp geometry and complexity 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 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.
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 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.
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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 Simple is better 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 similar to a diamond interchange and ramp to ramp routing should be enabled if possible and legal. However, in many SPUIs such ramp to ramp routing is not possible:
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 Freeway that serve to keep merging traffic out of the flow of through traffic on the mainline freeway.
Collector/distributor lanes serve as either:
- some of the ramps in an interchange, or
- local lanes in configurations with local-express lanes.
Collector/distributor interchange
Some interchange configurations make use of collector/distributor lanes to separate lower-speed merging traffic from high-speed through traffic. This is often used in cloverleaf interchanges and in groups of nearby exits.
Collector/distributor cloverleaf
This is a cloverleaf interchange that is connected to a collector/distributor instead of directly to the main roadway. Map collector-distributor cloverleaf ramps as you would any other ramp.
The detour prevention mechanism will discourage Waze from routing users onto the collector-distributor and back onto the freeway – as long as the city name on the freeway is the same (or set to "no city") before, throughout, and after the collector-distributor. Previously this feature was not available and the ramps were set up to restrict the through route. Some of these ramp configurations may still be set up that way, so they can now be configured as pictured above with the through route enabled.
Complex collector/distributor interchange
Where collector/distributor lanes are used as part of an interchange, use the Ramp type for the collector/distributor lanes. Name the ramp segments as you would any other ramp segment.
Ensure that a name on the Freeway segments is consistent before and after the collector/distributor lanes, so that the detour prevention mechanism will prevent Waze from routing users erroneously.
Local-express lanes
Local-express lanes are similar to collector/distributor interchange, but on a larger scale. While collector/distributor interchanges typically have an exit number or numbers, local-express lanes typically share the same name, differentiated by "Local" for the collector/distributor lanes and "Express" for the thru lanes.
A local-express lane configuration is not technically an "interchange"; however, since its physical characteristics are similar to those of a complex collector/distributor interchange, it is discussed here.
Where collector/distributor lanes are used as part of a local-express lane configuration,
- use the same type (most likely Freeway ) for the local lanes as is used for the express lanes, and
- name the road as it is signed: typically "[Name] Local [Direction]": for example, "I-96 Local W" for local lanes (and "I-96 Express W" for the corresponding express lanes).
See also
Review the Wikipedia article on road Interchanges for further information on this topic.