/**
  * Sets the distance matrix request travel mode.
  *
  * @param string $travelMode The distance matrix request travel mode.
  *
  * @throws \Ivory\GoogleMap\Exception\DistanceMatrixException If the travel mode is not valid.
  */
 public function setTravelMode($travelMode = null)
 {
     $travelModes = array_diff(TravelMode::getTravelModes(), array(TravelMode::TRANSIT));
     if (!in_array($travelMode, $travelModes) && $travelMode !== null) {
         throw DistanceMatrixException::invalidDistanceMatrixRequestTravelMode();
     }
     $this->travelMode = $travelMode;
 }