Detour Prevention Mechanisms
Waze routing includes a big detour prevention (BDP) mechanism to prevent routing off a highway/freeway only to be routed right back on the same highway/freeway. The assumption is that drivers will prefer to remain on the highway/freeway even if Waze calculates that taking a series of ramps or other collector/distributor roads is slightly faster or shorter. It is important to understand that Waze uses a penalty-based routing system. It computes the total penalties for a route and selects the one with the smallest total penalty (including time to drive the route). Therefore it is possible for big detour routes to occur when the direct route is extremely slow.
The Big Detour Penalty will be applied to a Detour (a series of segments leading off and then back onto the same highway) which has a Direct Route alternative (an alternate route to the possible detour that stays on the highway). The criteria listed below are the rules the waze back end reportedly uses to check for detours and search for any alternative direct routes to "stay on the same highway".
A Possible Detour is a series of segments in the middle of a route that appear to be a detour off a highway and then back onto the same highway. The last segment before the Possible Detour and the first segment after the Possible Detour must belong to the same "Road Type Group" as shown in the table below.
An Alternate Route is an alternative to a Detour or Possible Detour. It begins immediately after the last segment before the Possible Detour and ends immediately before the first segment after the Possible Detour. If it meets the criteria below for Direct Route, the Alternate Route is also a Direct Route.
- Definition of a Detour. A detour is a a possible detour that meets all of these criteria:
- Name Discontinuity
- The freeway/highway segments immediately before and immediately after the possible detour must share at least one street name among their primary and alternate names. Either segment may have the shared name as either a primary or an alternate name.
- The possible detour must include at least one segment (including the last segment of the possible detour) that is missing this shared street name (as both a primary name and alternate names).
- No Other Continuous Name - The possible detour segments must not all share any other street name with the both the freeway/highway segments immediately before and immediately after the possible detour.
- Road Type Discontinuity - The freeway/highway segments immediately before and immediately after the possible detour must be from the same Road Type Group as shown in the table below. The possible detour must include at least one segment not in that same Road Type Group.
- Minimum Length - The possible detour must be more than one segment long.
- Maximum Length - The possible detour must be shorter than the threshold length as shown in the table below.
- Name Discontinuity
- Definition of a Direct Route - A Direct Route is an Alternate Route that meets the criteria below:
- Existence: There must be an alternate route connecting the freeway/highway segments immediately before and immediately after the detour which does not use any of the segments used in the possible detour.
- Continuity The alternate route should have name and road type continuity. Because checking for alternate route continuity requires a lot of computer resources, the waze team has tried several different approaches, and will change the approach they use from time to time without notice. One of these may be in use at any time, but we are never sure which one
- Type Continuity - The freeway/highway segment immediately after the possible detour must be accessible from another segment of the same road type group. This "other segment" need not be part of the Alternate Route - current guidance Continuity - All segments of the Alternate route must have both name and type continuity with the segment immediately before and the segment immediately after the Possible Detour. - Speculative. Probable, but unconfirmed.
- Type Continuity - The last segment of the Alternate Route is of the same road type group as the first segment after the detour. - Possible (a less resource-intensive substitute for checking all segments of the alternate route
|Road Type Group||Threshold|
- If the first segment after the possible detour is the beginning of a freeway or highway then the first segment 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 criteria above.
- For divided roadways that have the same 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 will 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 will not use the U-Turn road because it 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.
- Be careful of "false positives!" For example, where route number concurrencies begin and end, the big detour mechanism may be unexpectedly triggered as follows (see diagram):
- In a concurrency, two highways share the same road, and both 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 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 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).
Waze has a small detour prevention mechanism to deal with instances where Waze calculates that 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 measurable difference in the route times. The exact difference in time required to trigger this prevention is proprietary, and subject to change as needed.