Difference between revisions of "Junction Style Guide/Interchange"
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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.
Rule #1 is stillsimpler 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.
Latest revision as of 04:11, 23 September 2019
An interchange is a road junction where traffic can move between roads that do not intersect. The roads are connected by ramps, and if they cross, the crossing is grade-separated. They are most commonly used where one or more roads is a controlled-access highway. Complex interchanges may contain many highways and local roads meeting within small areas. Many different layouts have been developed by traffic engineers to optimize interchanges for size, complexity, traffic safety, navigation, and unimpeded traffic flow.
This article is a sub-article of the Junction Style Guide. As such, this article is a style guide as well. Representing interchanges on the map can be exacting and difficult. The guidance on this page will help editors to create accurate and usable map versions of these interchanges. 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. Also note that 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.
Before reading through this article, be sure to fully understand the information in the Junction Style Guide.
- 1 Ramps
- 2 Component junction types
- 2.1 Basic exit/entrance
- 2.2 Complex exit
- 2.3 Freeway/highway fork
- 2.4 Ramp fork
- 3 Configurations
- 3.1 Diamond
- 3.2 Cloverleaf
- 3.3 Folded diamond
- 3.4 Single-point urban interchange (SPUI)
- 3.5 Collector/distributor lanes
- 3.6 Diverging diamond (DDI)
- 4 See also
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.
Thetype 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 thin but show at wide zoom levels, so interchanges do not distract from highways but can be seen at high speeds.
When to use ramps
Use of thetype is governed by the following rules:
Ramp geometry and complexity
This section concerns the geometry of ramps at basic exits and entrances, which are the majority of use cases for ramps. For information on the geometry of other junctions involving ramps, see the other sections below.
When placing the beginning of an exit ramp, first place a geometry node of the ramp segment at the nearest point to the exit from among:
- If there's no solid white line, at the gore point (where the painted lines diverge)
- If there's a solid white line, at its beginning
- The beginning of the inside solid white line on a multi-lane exit
- 1/4 mile before the gore point, on exits with a longer solid white line
- Halfway between the gore points of the exit and the previous exit
Extend the ramp naturally to create a junction with the road segment at a 20° departure angle. This will allow for consistent timing of exit instructions and make it easier to report closures in the Waze client. At the end of an entrance ramp place a geometry node at the end of the solid white line and extend the ramp to join with the highway at a reasonable departure angle that follows the natural road geometry. The angle used should allow for smooth and 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 , , and
- Grade-separated cross streets which may be highways or lower road types such as
- 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.
A basic exit is a junction where a single ramp exits a highway and leads to another road, and the continuation of the highway is obvious to drivers. Most exits fit this description.
To map a basic exit:
- The entering segment and the continuing segment must be , , or type and should match name and type.
- The freeway/highway continuing segment should have close to a zero degree departure angle from the entering segment.
- The exiting segment should be of the type .
- The ramp geometry should be consistent with ramp geometry and complexity.
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 "keep 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.
Similarly, a basic entrance is where a single ramp joins an existing highway, and the path of the highway is obvious to drivers. The geometry of a basic entrance junction should be mapped just like a basic exit, but with directions reversed.
Basic exit geometry
See ramp geometry and complexity for information on setting the angles of basic exits.
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 the section on exit ramp names. In addition, if an exit carries a concurrent route away from the highway, for example a US route that was carried by an interstate up to the exit but splits off at the exit, that route designation should be added as an alternate name on all ramp segments that carry it.
A complex exit is a junction where one or more exit ramps leave the highway, and the continuation of the highway is not obvious to drivers. Therefore, another instruction is needed for traffic continuing on the highway. This can be a "keep" or "continue" instruction. When mapping complex exits, the geometry should match the instruction given, so for "continue" instructions the geometry should be set up as described for exits in ramp geometry and complexity, and for keep instructions (i.e. most wayfinder exits), the geometry should be set up in the same way as a freeway fork. For criteria and further details on mapping complex exits, see wayfinder and turn instruction override.
A highway/freeway fork is a junction where one freeway or highway splits into two, and there is no obvious straight through direction for a driver. This can happen at the end of a concurrency of highway routes or where one or both highway routes begin. This is synonymous with the MUTCD term "split."
Freeway fork geometry
Place the first geometry node of each outgoing segment at the beginning of the solid white line where the roadways diverge, then move the junction node to where each outgoing segment deviates from the incoming segment 20° right and left.
Freeway fork naming
The purpose of freeway fork naming is to give instructions for both right and left side of the fork that are accurate to posted signage. That can be accomplished using the following:
- Signs and wayfinder stub segments: If an outgoing segment has a big green sign (BGS) above it that says something different than the name of the entering segment, cut the segment at the fork to create a stub and name it according to what is on the sign. If the outgoing segment continues the route designation of the incoming segment, add that as an alternate name of the stub to preserve name continuity for detour prevention purposes. If the BGS doesn't say anything different from the name of the highway itself, there is no need to make a stub; simply name the outgoing segment with the name of the highway.
- A turn instruction override: If one branch is named the same as the entering segment either as a primary or alt name, use a turn instruction override to give a "keep left" or "keep right" instruction depending upon which direction the branch leaves the fork.
For further details on freeway fork mapping, see the wayfinder page.
A ramp may itself fork and branch into two directions. This is synonymous with the MUTCD term "bifurcation." Most of the time a ramp fork should be mapped to give instructions to traffic going either direction. This is accomplished by using the ramp type for both outgoing segments and names that are different from the incoming segment or turn instruction overrides where necessary.
Ramp fork geometry
Ramp forks should generally have the same geometry as freeway forks, except for where the ramp fork is part of a collector/distributor interchange or if one side of the ramp fork is using a "continue" instruction or no instruction. In this case the ramp fork should look more like a basic exit.
Ramp fork naming
Naming of ramp forks should generally follow the guidelines from road names, but there are multiple ways to handle all the signs present at each junction.
Using information on the signs directly
A simple way to name the ramp segments in ramp forks is to name each ramp segment with full or abbreviated information shown on the signs leading into it. This will provide a static set of instructions for users as they travel on each segment, regardless of what their further movements will be. This is especially appropriate when a single sign is present at an exit, but multiple signs with different information are present farther down the road.
Using road name inheritance
Another way to name these ramp segments is to use name inheritance. If a ramp is unnamed ("no name" box checked), the name of a subsequent ramp on the route will propagate backwards in navigation instructions. This is useful both for the sake of simplicity and for giving more specific instructions to traffic at exits with ramp forks. If an unnamed ramp is used at an exit and subsequent named ramps are used after the fork, drivers will only see the name of whichever side of the fork they need to go to before they exit the highway. This method will provide more sufficient notification of an approaching decision point than a named exit ramp would, and it should be used as long as the names of both ramp forks are visible on signs at the start of the initial ramp. If an exit ramp has multiple lanes with a sign or part of a sign over each lane, using this method can even function as a form of lane guidance. If the example on the right from the MUTCD were mapped using name inheritance, the ramp exiting I-42 would not be named. The ramp that goes to I-17 southbound would be named "Exit 36: I-17 S / Portland" and the ramp that goes to I-17 northbound would be named "Exit 36: I-17 N / Miami." This would produce the following instructions:
- Traffic heading south on I-17 would receive
- at the exit: exit right to Exit 36: I-17 S / Portland
- at the fork: keep right to Exit 36: I-17 S / Portland
- Traffic heading north on I-17 would receive
- at the exit: exit right to Exit 36: I-17 N / Miami
- at the fork: keep left to Exit 36: I-17 N / Miami
Note that even though the exit number is by design not shown on signs at the ramp fork, it should be included in the names of the ramps for proper instructions at the exit. If signs at the ramp fork differ more significantly from signs at the exit, a different method of naming should be used.
Name inheritance, but signage on consecutive signs are different
If separate or split signs exist for traffic at an exit, but the signs at the ramp fork differ significantly from them, such as being further split or showing additional route numbers or control cities, the following method can be used:
- Leave the exit ramp unnamed
- At the ramp fork create a turn instruction override for no instruction going into a stub ramp segment of 19.69 ft (6 m)
- Name the stub according to the sign at the exit
- At the junction of the stub with the next ramp segment create a turn instruction override to match the expected instruction at the ramp fork, either keep left or right
- Name the next ramp segment according to the sign at the ramp fork or leave it unnamed to inherit farther ramp names
Because of name inheritance, the shortness of the stub, and the combination of turn instruction overrides, the name of the stub will be used in instructions at the exit, and the name of the ramp past the stub will be used at the ramp fork. This method should only be used when it's not possible to replicate what drivers see on guide signs using simple naming or name inheritance.
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.
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. Use turn instruction overrides from the entrance ramp to give no instruction to the freeway and an exit instruction to the exit.
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 easy restriction of 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 Simple is better section 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 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 four 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 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.
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.
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 street name on the freeway is the same 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 thetype 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 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 ) 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).
Diverging diamond (DDI)
See also: Diverging Diamond Interchange article on Wikipedia.
Diverging diamond interchanges (DDI) are a type of diamond interchange in which the two directions of traffic cross one another on each side of a limited-access roadway. A DDI may pass over or under the limited-access roadway.
This type of interchange is unusual, in that it requires traffic to briefly drive on the opposite side of the road from what is customary for the jurisdiction. However, the design of the Diverging Diamond Interchange controls the driver's line of sight to ensure the cross-over action feels natural and goes unnoticed.
All ramp and surface street segments are set as one-way. If you are creating a DDI along a road which is not divided, divide the road, first.
- For more details on how to properly divide/un-divide a road, see Best map editing practice § Dividing and un-dividing divided highways.
As with all at-grade intersections in Waze, all DDI at-grade intersections are modeled with junction nodes, including the two signaled intersections where opposing directions of traffic "cross over" each other (inner surface road junctions). A DDI may also have two outer surface road junctions, where the one-way segments transition to two-way road segments.
There are four junctions in a DDI at which the turn restrictions must be checked - two inner surface road junctions where traffic crosses, and two outer surface road junctions where the road divides/joins on each side of the DDI.
Inner surface road junctions
Disable the two turns from one-way segments to the segments carrying traffic the opposite direction at both inner surface road intersections, for a total of four disabled turns.
Outer surface road junctions
Disable the single turn from the one-way segment carrying traffic exiting the DDI to the one-way segment carrying traffic entering the DDI at both outer surface road intersections, for a total of two disabled turns.
Review the Wikipedia article on road Interchanges for further information on this topic.