public static function save_taxonomy_term( $post_id, $taxonomy_name, $element_name ) {
		if ( ! isset( $_POST[ $element_name ] ) ) {
			return;
		}

		$term_slug = sanitize_text_field( wp_unslash( $_POST[ $element_name ] ) );
		if ( '' === $term_slug ) {
			// the user is setting the vehicle type to empty string.
			wp_remove_object_terms( $post_id, self::get_term_slug( $taxonomy_name, $post_id ), $taxonomy_name );
			return;
		}
		$term = get_term_by( 'slug', $term_slug, $taxonomy_name );
		if ( empty( $term ) || is_wp_error( $term ) ) {
			// the term does not exist. create it.
			$term_arr = array(
				'slug'        => sanitize_title( $term_slug ),
				'description' => $term_slug,
				'name'        => $term_slug,
			);
			$id_arr   = wp_insert_term( $term_slug, $taxonomy_name, $term_arr );
			if ( ! is_wp_error( $id_arr ) ) {
				$term = WP_Term::get_instance( $id_arr['term_id'], $taxonomy_name );
			}
		}
		$set = wp_set_object_terms( $post_id, $term->term_id, $taxonomy_name, false );
	}