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Detour Prevention Mechanisms

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Update for clarification of name continuity
;Alternate route :What Waze compares a route to - when evaluating a detour route, the direct route is the evaluated alternative.
;Road type group continuity :When a set of continuous segments all belong to the same road type group (see table below), there is '''road type group continuity'''. '''Road type group discontinuity ''' occurs when one of the segments in the set of continuous segments belongs to a different road type group.
;Name Street name continuity :When a set of continuous segments all share a street name, there is '''street name continuity'''. It doesn't matter whether the street name is found as a primary or alternate name on any of the segments, as long as it is present in each. For the purposes of detour prevention, only the name of the street itself must match. The city name can be different, and it will still be considered continuous. '''Name Street name discontinuity''' occurs when one or more of the segments in this continuous set does not share the street name shared by all the other segments.
Waze makes a few checks on an off-highway '''detour''' to see if it should be prevented. It checks the length of the '''detour, '''makes sure the ''road type group'' changes, and that the street name is ''not'' continuous with the highway the route detours from. It also checks that there exists an '''alternate route''' that is direct - the '''direct route''' must have street name continuity and road type group continuity (see details below). Waze does not consider how many 'turn...', 'keep...', or 'exit...' instructions are issued on any of the paths it checks for detour prevention.
=== {{@|Criteria}} ===
If a '''detour''' being examined passes the checks listed below, we'll call it a '''preventable detour. '''If a route leaves a highway (based on road group type) to one or more segments whose road type is not in the same '''road type group''' as the highway, only to re-enter that same highway, Waze will look for '''alternate routes''' that get to that same point try to find a '''direct route'''.''' '''If a '''direct route '''is found, then Waze will apply the big detour prevention (BDP) [[Routing penalties|penalty]] to the '''preventable detour '''favoring the''' direct route'''. Here are the specific checks that Waze makes:
<ol style="list-style-type: upper-roman;"><li>'''Bracketing segments''' '''(segment immediately before and immediately after the detour):''' The freeway/highway bracketing segments must share at least one street name among their primary and alternate names(city names are not required to match). Either segment may have the shared street name as either a primary or an alternate name. These segments must also be from the same '''road type group''' as shown in the table below. The road type group of these segments determines the maximum length of detour for which BDP will be applied. </li>
<li>'''Preventable detour:''' A preventable detour is a possible detour that meets all of these criteria:
<ol style="list-style-type: lower-alpha;"><li>'''Name Street name discontinuity:''' There must be a break in the '''street name continuity''' between the detour and the bracketing segments. {{As of|February 2018}} only the last segment of the detour is checked. This segment must not share any street names with the highway segment just after the detour (AKA 2nd '''bracketing segment'''). </li>
<li>'''Road type group discontinuity:''' There must be a break in '''road type group''' between the detour and the bracketing segments. {{As of|February 2018}} only the last segment of the detour is checked. This segment must have a different road type group than the highway segment just after the detour (AKA 2nd '''bracketing segment''').</li>
<li>'''Minimum length:''' The possible detour must be more than one segment long.</li>
<li>'''Direct route:''' A direct route is an alternate route that meets the criteria below:
<ol style="list-style-type: lower-alpha;"><li>'''Existence''': There must be a different way to go - an '''alternate route''' - connecting the freeway/highway '''bracketing segments''', which does not use any of the segments in the detour.</li>
<li>'''Continuity:''' The entire direct route should have '''street name continuity''' and '''road type group continuity'''. {{As of|2017|12}}, this is done with a '''full continuity check''', meaning all segments of the '''alternate route''' must have both road type group and street name continuity with the '''bracketing segments'''. This is the most resource-intensive continuity check, but is also the most robust way to to help ensure we don't penalize routes unless there actually is a comparable route that doesn't involve changing roads at all. </li></ol>
</li>
</ol>
=== {{@|Notes}} ===
* If the first segment after the detour is the beginning of a freeway or highway, and thus can only be accessed from ramps or segments of a different road type group, then there can be no direct route according to the above criteria.
*For divided roadways that have the same street name on both carriageways, the big detour prevention mechanism will prevent U-turns that traverse more than one segment. Thus, in the intersection below, the configuration on the left should be used to route a U-turn because it uses a single segment between the two parallel roadways with the same name. In contrast, the configuration on the right should not be used because the U-turn road branches from another segment causing multiple segments between the two parallel roads with the same name, and the U-turn is not in the same road type group, preventing BDP from working.
[[File:UDetour.png|border|center]]
* Be careful of "false positives!" For example, where route number concurrences begin and end, the big detour prevention mechanism may be unexpectedly triggered as follows (see diagram):
** In a concurrency, two highways share the same road, and both street names will appear on the concurrent segments, using [[alternate names]]. Where the concurrency ends, the two highways split into separate roadways again.** Often, there are ramp segments connecting the last concurrent segment to one or both of the non-concurrent continuations. If the concurrency was of I-1234 and I-4567, for example, there might be a ramp named "to I-1234 / Sometown" - a street name which contains "I-1234" but is not identical to the highway name.
** If the ramp does not also carry a simple alternate name of "I-1234", then there is a discontinuity of the highway name between the concurrency and the continuation highway. If there are two ramp segments like this, it will trigger Big Detour Prevention unexpectedly, and Waze will create an unneeded penalty for continuing on I-1234.
** To prevent this, just assign I-1234 as an alternate street name to all ramps between the two sections (concurrency and continuation). Note that if there is only a single ramp segment connecting the highways, there will not be a "false positive." Nevertheless, it is a good practice to do this even for a single ramp segment, in case the ramp is cut into two segments later, either by accident or because later construction or map improvements connect another feeder ramp in the middle.
* Another example of a false positive could occur where a highway has multiple wayfinders connected, one to another, (for example, if there are several multi-lane exits in a row).
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