Detour Prevention Mechanisms

Waze searches for the fastest route to the destination, but sometimes, a promising route takes detours from the most obvious, most direct, or otherwise most continuous route. These detours may involve taking side streets, making a few extra turns, or taking an exit off a highway and then re-entering the same highway a short distance later. Unless the detours actually save a notable amount of time, they aren't worth it to drivers, and Waze shouldn't suggest them. Here are two cases where Waze recognizes a detour in the route and applies a penalty to prevent that detour unless it saves enough time to be "worth it."

Big detours

The big detour prevention (BDP) is a penalty meant to keep Waze from telling you to detour off a highway and then get right back on it unless it saves you enough time to be "worth it" - for example, if traffic is really slow on the "direct" route.

Direct route

The continuous route most users would expect. For example, a route that stays on the same freeway from point A to point B.

Detour

Any route used in place of the direct route.

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.

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. 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) penalty to the preventable detour favoring the direct route. Here are the specific checks that Waze makes:

1. 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.
2. Preventable detour: A preventable detour is a possible detour that meets all of these criteria:
   1. 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).
   2. 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).
   3. Minimum length: The possible detour must be more than one segment long.
   4. Maximum length: The possible detour must be shorter than the threshold length as shown in the table below for the road type group of the bracketing segments..
3. Direct route: A direct route is an alternate route that meets the criteria below:
   1. 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.
   2. Continuity: The entire direct route should have street name continuity and road type group continuity. As of December 2017, 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.

Road Type Group	Threshold
Freeway   Major Highway	5km
Minor Highway	500m

Flowchart Waze Editors can use to check whether Big Detour Prevention Penalty should be applied to a route

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.

• 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).

Small detours

Waze has a small detour prevention (SDP) mechanism to deal with instances where Waze calculates that a left turn can be "optimized" by using U-turns and right turns (either continue straight, U-turn, right turn; or alternatively, immediate right turn, U-turn, continue straight). While this may save a few seconds over waiting for a long average left turn, it is undesirable. Waze will prevent such detours if there is not a notable difference in the route times. The actual difference in time required to trigger this prevention is a closely-guarded secret, but if you come across one of these where you feel Waze didn't adequately prevent it, please contact your community leadership for advice. There may still be a solution, but it may have to be evaluated on a case-by-case basis.