Although we regularly benchmark our models against other providers including Google to ensure that the results are comparable, there are a number of important reasons why they can differ.
Public Transport
Different transport agencies
Google are not transparent about what public transport agencies they use, so it is hard to compare directly, but we publish our data here (get in touch with [email protected] for a full list of agencies)
In benchmarking exercises we sometimes find that we have public transport routes not included in Google
Google therefore may return a slower public transport route as it lacks the data of a faster alternative
Different default parameters
The following parameters are all completely configurable using the TravelTime API, but with the Google API they are not, and it isn't clear what the default values are:
Maximum Walking Time - the time allowed to walk from the origin to the first public transport stop/station and from the final stop/station to the destination. Often Google will return a result that requires more than the default Maximum Walking Time for the TravelTime API (15 minutes)
Change Delay - the time penalty (if any) applied when changing from one public transport leg to another
Maximum Changes - the number of changes allowed during the journey
Different logic
If there is no route available at the arrival / departure time selected, Google will try to return an alternative (such as a journey that is actually the night before), whereas the TravelTime API will simply return an unreachable result
Driving
The TravelTime driving model does not take into account live traffic conditions, but instead is modelled around expected congestion at a particular time of the day
Different default parameters
The following parameters are all completely configurable using the TravelTime API, but the default values can cause differences with Google behaviour:
Snapping Penalty - when a driving journey starts away from the road network, the default TravelTime behaviour is to apply a time penalty based on how long it would take to walk to the nearest road, based on an average walking speed. Google will snap to the network without a time penalty.
Max Snapping Distance - the default maximum distance that a journey can start from a road is 1km when using the TravelTime API, whereas it is 5km when using Google.