public DistanceMatrixApiRequest trafficModel(TrafficModel trafficModel) {
    return param("traffic_model", trafficModel);
  }
Specifies the assumptions to use when calculating time in traffic. This parameter may only be
specified when the travel mode is driving and the request includes a departure_time.

@param trafficModel The traffic model to use in estimating time in traffic.
@return Returns this {@code DistanceMatrixApiRequest} for call chaining.
src/main/java/com/google/maps/DistanceMatrixApiRequest.java