User:CBenson/Detour Prevention Mechanisms

Big Detours

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.

Criteria

A "possible detour" is a series of segments in the middle of a route which meet the minimum criteria to trigger an evaluation.

A "detour" is defined as a portion of a route which meets all the criteria listed below.

Possible detours are evaluated to determine if they meet ALL the following criteria, and are penalized if they do. NOTE: If the continuation path we wish Waze to use also meets the below criteria it will be equally penalized, cancelling the BDP effect.

1. Road Name Discontinuity - The freeway/highway segments immediately before and immediately after the possible detour must share at least one street name (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).
   1. 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.
2. Road Type Discontinuity - The freeway/highway segments immediately before and immediately after the possible detour must be from the same 'Road Type Group' as defined in the chart below. The possible detour must include at least one segment not in that same 'Road Type Group.'
3. Existence Of Direct Route - The BDP mechanism is intended to penalize detours away and back to a direct route. The following two criteria are used to determine if there may be a possible direct route.
   1. Existence Of Alternate Route - There must be an alternate route connecting the freeway/highway segments immediately before and immediately after the possible detour which bypasses the segments of the possible detour.
   2. Alternate Road Type Continuity - The freeway/highway segment immediately after the possible detour must be accessible from another segment of the same road type group.
   • The BDP mechanism does not determine that there is actually a direct route with name and type continuity, but rather only uses the above two criteria.
   • Thus, the alternate route may be a detour itself. The alternate route need only use different segments than the possible detour being evaluated.
   • If the first segment after the possible 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, it is assumed that there is no direct route.
4. Minimum Length - The possible detour must be more than one segment long.
5. Maximum Length - The possible detour must be shorter than the threshold length as defined in the chart below.

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

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 least cumulative penalty. Therefore it is possible for big detour routes to occur when the primary route is extremely slow.

Notes

• 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).

Small Detours

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.