JQuery - json por jquery acceso a datos array de datos

<?php

mysql_query('SET CHARACTER SET utf8');

    $res = mysql_query("SELECT * FROM mi_tabla ORDER BY en_este_caso_id");

    $array = array();

    while($obj = mysql_fetch_object($res)) {

        $array []= $obj;

    }

    // echo '' . json_encode($array, JSON_UNESCAPED_UNICODE) . '';

    file_put_contents("rutaq_al_archivo.json", json_encode($array, JSON_UNESCAPED_UNICODE));

?>

[{"idruta":"00000002","ruta":"VGBAR01","origen":"Vitoria-Gasteiz, Álava, España","latorigen":"42.859653","lngorigen":"-2.681649","destino":"Barcelona, España","latdestino":"41.385048","lngdestino":"2.173416","waypoints":"(42.8405390,-1.6829537),(41.7870155,-1.2452868),(41.5577848,0.5719991),(41.1360642,1.1013598)","travelMode":"DRIVING","avoidHighways":"false","avoidTolls":"true"},{"idruta":"00000003","ruta":"BIOVIG01","origen":"Bilbao, Vizcaya, España","latorigen":"43.256706","lngorigen":"-2.924113","destino":"Vigo, Pontevedra, España","latdestino":"42.240616","lngdestino":"-8.720695","waypoints":"{43.3109587,-3.9243324},{42.4496043,-3.4831784},{41.6609780,-4.6865778},{42.5856973,-5.5509483},{42.9815923,-7.5619522}","travelMode":"DRIVING","avoidHighways":"true","avoidTolls":"false"}]

alert (data[i].waypoints.length);

[{... , ... ,"waypoints":"[42.8405390,-1.6829537],[41.7870155,-1.2452868],[41.5577848,0.5719991],[41.1360642,1.1013598]",... , ,,,},{... , ... ,"waypoints":"[43.3109587,-3.9243324],[42.4496043,-3.4831784],[41.6609780,-4.6865778],[42.5856973,-5.5509483],[42.9815923,-7.5619522]",,... , ,,,}]

$.getJSON('ruta_archivo.json', function (data) {

    for (var i = 0; i < data.length; i++) {

/* ... */

          var waypoints = eval ( '['  + data[i].waypoints +  ']' );

/* Ahora sí alert (waypoints.length); muestra como array con pares de campos tales

como [42.4496043,-3.4831784] que los recojo como waypoints[j][0] y waypoints[j][1] */

          var wp = [];

		  for (var j = 0; j < waypoints.length; j++){

		    wp.push({'location': new google.maps.LatLng(parseFloat(waypoints[j][0]),parseFloat(waypoints[j][1])),'stopover':false});

/* Esta parte corresponde al cógido especifico de mi interés para google maps (waypoints) */

		  }

/* ... */

    }

  });