== Real-time current road speeds ==
The routing server uses real-time road speeds (from recent Waze app users if available) and combines the historical speed of the segment broken down into 10-15 minute chunks. The time to pass through a segment is tracked separately for each route out of a segment. For example if a segment ends with a left and right turn (no straight let's say), then the routing engine isolates the time through the segment to turn left and the time through the segment to turn right
. This detailed speed information is proprietary & is part of the data Waze sells/plans to sell and won't be displayed through the editor.
It is known that Waze uses real-time reports of current road speeds by preference over historical average road speeds. Waze also uses traffic congestion reports to reroute around slow traffic. <!-- Early versions of the client would also report when traffic speeds were good - this was removed due to map clutter. But Waze will still be receiving this information. -->It is assumed that the speed of any Wazer ahead of you on a stretch of road will be the speed used when your route is calculated.
As the number and density of Wazers grows, this real-time data takes on a greater importance. This emphasis is partly a reflection of Waze's original goal to create optimal commutes. As Waze has grown to be used as a more general purpose GPS navigation device over roads less traveled, the historical average road speeds becomes more important.
Since Waze uses the time you expect to be at a road segment to calculate the expected speed on that segment, it cannot use real time data when your trip time moves into a future time slot. So trips
of more than 30 minutes (and on average more than 15 minutes) will include future time slots where current traffic data is not available when initially calculated. When you enter a new time slot, any real-time traffic data that is available causes your client to recalculate your ETA and reroute you at that point.
== Fastest routing ==
The following are reasons why the routing engine might not use the average road speed.
=== Time of day variations ===
Consider a road that most Wazers drive at 5pm when the average speed is 12 mph. You choose your route at 10am when the road is clear and the average speed is 60mph. It is the best road to take, but Waze chooses a different route because it has no information about the speed at 10am and assumes the speed is 12mph all day.
You can, of course, drive the road yourself, and Waze eventually
learns the speed for that time. We do not know how long Waze holds the older data in computing the average speed. However, if Waze does discard old time information, you might not accumulate new time faster than it is discarded. It is also possible that your times are being ignored as being abnormal.
More Waze users would help fix this.
Waze does not record the location of traffic lights. While some GPS navigation offers guidance like "turn right at the next traffic light" the information is frequently incomplete, incorrect or outdated. The consensus view is that Waze should not record the location of traffic lights.
Waze does take traffic lights and stop signs into account by noting the effect they have on traffic speed. Consider a traffic light with long waiting times. The road segment leading to that traffic light will have a low average speed. If the average speed (based on the average waiting time) becomes low enough, a longer route that avoids the light becomes the preferred route. This has been observed in practice and is an example of ''emergent
behaviour''. Waze isn't programmed to avoid traffic lights but it does avoid slow roads; if the traffic lights make the road slow then Waze avoids them.
Some drivers regularly take longer routes — even winding through side streets — to avoid any stops or traffic lights. Waze has been known to suggest this, and also known to revert to waiting at lights when better average speed information is collected from the side streets. But note that this can be less than optimal due to the turn delays discussed above.