public DirectionsApiRequest mode(TravelMode mode) {
    return param("mode", mode);
  }
Specifies the mode of transport to use when calculating directions. The mode defaults to
driving if left unspecified. If you set the mode to {@code TRANSIT} you must also specify
either a {@code departureTime} or an {@code arrivalTime}.

@param mode The travel mode to request directions for.
@return Returns this {@code DirectionsApiRequest} for call chaining.
src/main/java/com/google/maps/DirectionsApiRequest.java