# Detour Prevention Mechanisms

## Big Detours

Waze routing includes a 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

Waze routing will reduce the chances of a detour off a highway/freeway and back on that same highway/freeway when all of the following conditions are met:

A "detour" is defined as a series of segments in middle of a route which do not contain the same name as segments on either side of them in the same route.

NOTE: The continuation path we wish Waze to use must not also meet the below criteria, or it will be equally penalized, cancelling the BDP effect.

1. The highway / freeway segments both before and after the detour need to have the same (primary, alternate, or any combination of both) street name, which is not present on the detour segments. As of June 2015 city names do not affect this algorithm. While they used to in the past, the algorithm has been updated.
2. The detour is not the same 'road type group' as the continuation path. Road type groups are listed in the chart below.
3. The detour is more than one segment long, and
4. The detour is under the threshold length as defined in the chart below.

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.