TrafficModel

This field specifies one of the following assumptions to use when calculating travel time in traffic conditions, shown in the enums below. Depending on the enum chosen, the duration field of the TrafficModel response will vary. The value contains the predicted time to destination in traffic, based on historical averages. TrafficModel is only available for requests that have set RoutingPreference to TRAFFIC_AWARE_OPTIMAL and RouteTravelMode to DRIVE.

Enums
TRAFFIC_MODEL_UNSPECIFIED Unused. If specified, will default to BEST_GUESS.
BEST_GUESS Indicates that the returned duration should be the best estimate of travel time given what is known about both historical traffic conditions and live traffic. Live traffic becomes more important the closer the departureTime is to now.
PESSIMISTIC Indicates that the returned duration should be longer than the actual travel time on most days, though occasional days with particularly bad traffic conditions may exceed this value.
OPTIMISTIC Indicates that the returned duration should be shorter than the actual travel time on most days, though occasional days with particularly good traffic conditions may be faster than this value.